それぞれのバージョン

Ruby 2.6.3
Rails 6.0.2.2
Homebrew 2.2.14
MySQL 8.0.19

bundle installでエラーが出る！

$ bundle install
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
Fetching gem metadata from https://rubygems.org/............
Using rake 13.0.1
Following files may not be writable, so sudo is needed:
  /Library/Ruby/Gems/2.6.0
  /Library/Ruby/Gems/2.6.0/build_info
  /Library/Ruby/Gems/2.6.0/build_info/mysql2-0.5.3.info
  /Library/Ruby/Gems/2.6.0/cache
  /Library/Ruby/Gems/2.6.0/doc
  /Library/Ruby/Gems/2.6.0/extensions
  /Library/Ruby/Gems/2.6.0/gems
  /Library/Ruby/Gems/2.6.0/specifications
Using concurrent-ruby 1.1.6
Using i18n 1.8.2
Using minitest 5.14.0
Using thread_safe 0.3.6
Using tzinfo 1.2.7
Using zeitwerk 2.3.0
Using activesupport 6.0.2.2
Using builder 3.2.4
Using erubi 1.9.0
Using mini_portile2 2.4.0
Using nokogiri 1.10.9
Using rails-dom-testing 2.0.3
Using crass 1.0.6
Using loofah 2.5.0
Using rails-html-sanitizer 1.3.0
Using actionview 6.0.2.2
Using rack 2.2.2
Using rack-test 1.1.0
Using actionpack 6.0.2.2
Using nio4r 2.5.2
Using websocket-extensions 0.1.4
Using websocket-driver 0.7.1
Using actioncable 6.0.2.2
Using globalid 0.4.2
Using activejob 6.0.2.2
Using activemodel 6.0.2.2
Using activerecord 6.0.2.2
Using mimemagic 0.3.4
Using marcel 0.3.3
Using activestorage 6.0.2.2
Using mini_mime 1.0.2
Using mail 2.7.1
Using actionmailbox 6.0.2.2
Using actionmailer 6.0.2.2
Using actiontext 6.0.2.2
Using public_suffix 4.0.4
Using addressable 2.7.0
Using bindex 0.8.1
Using msgpack 1.3.3
Using bootsnap 1.4.6
Using bundler 1.17.2
Using byebug 11.1.3
Using regexp_parser 1.7.0
Using xpath 3.2.0
Using capybara 3.32.1
Using childprocess 3.0.0
Using ffi 1.12.2
Using jbuilder 2.10.0
Using rb-fsevent 0.10.3
Using rb-inotify 0.10.1
Using ruby_dep 1.5.0
Using listen 3.1.5
Using method_source 1.0.0
Fetching mysql2 0.5.3



Your user account isn't allowed to install to the system RubyGems.
  You can cancel this installation and run:

      bundle install --path vendor/bundle

  to install the gems into ./vendor/bundle/, or you can enter your password
  and install the bundled gems to RubyGems using sudo.

  Password: 
Installing mysql2 0.5.3 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

current directory:
/private/var/folders/wk/48tb0_tn1q32bdgs7gkj7zjh0000gn/T/bundler20200507-40003-1nb47t8mysql2-0.5.3/gems/mysql2-0.5.3/ext/mysql2
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby -I
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0 -r
./siteconf20200507-40003-fqzid1.rb extconf.rb
--with-ldflags\=-L/usr/local/opt/openssl/lib\
--with-cppflags\=-I/usr/local/opt/openssl/include
checking for rb_absint_size()... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/$(RUBY_BASE_NAME)
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:467:in
`try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:546:in
`block in try_link0'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/tmpdir.rb:93:in
`mktmpdir'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:543:in
`try_link0'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:570:in
`try_link'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:782:in
`try_func'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:1069:in
`block in have_func'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:959:in
`block in checking_for'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:361:in
`block (2 levels) in postpone'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:331:in
`open'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:361:in
`block in postpone'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:331:in
`open'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:357:in
`postpone'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:958:in
`checking_for'
from
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:1068:in
`have_func'
	from extconf.rb:25:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found
here:

/var/folders/wk/48tb0_tn1q32bdgs7gkj7zjh0000gn/T/bundler20200507-40003-1nb47t8mysql2-0.5.3/extensions/universal-darwin-19/2.6.0/mysql2-0.5.3/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in
/var/folders/wk/48tb0_tn1q32bdgs7gkj7zjh0000gn/T/bundler20200507-40003-1nb47t8mysql2-0.5.3/gems/mysql2-0.5.3
for inspection.
Results logged to
/var/folders/wk/48tb0_tn1q32bdgs7gkj7zjh0000gn/T/bundler20200507-40003-1nb47t8mysql2-0.5.3/extensions/universal-darwin-19/2.6.0/mysql2-0.5.3/gem_make.out

An error occurred while installing mysql2 (0.5.3), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.5.3' --source 'https://rubygems.org/'` succeeds
before bundling.

In Gemfile:
  mysql2

こんなエラーが出てきました。↓

An error occurred while installing mysql2 (0.5.3), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.5.3' --source 'https://rubygems.org/'` succeeds
before bundling.

ということでわかってないけど言われた通りにやってみる。
はじかれたので　sudo　をつけました。

$ sudo gem install mysql2 -v '0.5.3' --source 'https://rubygems.org/'
Password:
Building native extensions. This could take a while...
ERROR: Error installing mysql2:
ERROR: Failed to build gem native extension.

またエラー。

#解決策

結局このコードで解決しました。↓

bundle config --local build.mysql2 "--with-ldflags=-L/usr/local/opt/openssl/lib"

なんでできるのかはわかってないので（←オイ）
こちらの記事を参考にしてくださいw　↓
https://qiita.com/corocn/items/fa51760fdd01c2c6854e

$ bundle config --local build.mysql2 "--with-ldflags=-L/usr/local/opt/openssl/lib"
You are replacing the current local value of build.mysql2, which is currently "--with-ldflags=-L/usr/local/opt/openssl/lib --with-cppflags=-I/usr/local/opt/openssl/include"
asatokensei@mba tomalog % bundle install
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
Fetching gem metadata from https://rubygems.org/............
Using rake 13.0.1
Following files may not be writable, so sudo is needed:
  /Library/Ruby/Gems/2.6.0
  /Library/Ruby/Gems/2.6.0/build_info
  /Library/Ruby/Gems/2.6.0/build_info/mysql2-0.5.3.info
  /Library/Ruby/Gems/2.6.0/cache
  /Library/Ruby/Gems/2.6.0/doc
  /Library/Ruby/Gems/2.6.0/extensions
  /Library/Ruby/Gems/2.6.0/gems
  /Library/Ruby/Gems/2.6.0/specifications
Using concurrent-ruby 1.1.6
Using i18n 1.8.2
Using minitest 5.14.0
Using thread_safe 0.3.6
Using tzinfo 1.2.7
Using zeitwerk 2.3.0
Using activesupport 6.0.2.2
Using builder 3.2.4
Using erubi 1.9.0
Using mini_portile2 2.4.0
Using nokogiri 1.10.9
Using rails-dom-testing 2.0.3
Using crass 1.0.6
Using loofah 2.5.0
Using rails-html-sanitizer 1.3.0
Using actionview 6.0.2.2
Using rack 2.2.2
Using rack-test 1.1.0
Using actionpack 6.0.2.2
Using nio4r 2.5.2
Using websocket-extensions 0.1.4
Using websocket-driver 0.7.1
Using actioncable 6.0.2.2
Using globalid 0.4.2
Using activejob 6.0.2.2
Using activemodel 6.0.2.2
Using activerecord 6.0.2.2
Using mimemagic 0.3.4
Using marcel 0.3.3
Using activestorage 6.0.2.2
Using mini_mime 1.0.2
Using mail 2.7.1
Using actionmailbox 6.0.2.2
Using actionmailer 6.0.2.2
Using actiontext 6.0.2.2
Using public_suffix 4.0.4
Using addressable 2.7.0
Using bindex 0.8.1
Using msgpack 1.3.3
Using bootsnap 1.4.6
Using bundler 1.17.2
Using byebug 11.1.3
Using regexp_parser 1.7.0
Using xpath 3.2.0
Using capybara 3.32.1
Using childprocess 3.0.0
Using ffi 1.12.2
Using jbuilder 2.10.0
Using rb-fsevent 0.10.3
Using rb-inotify 0.10.1
Using ruby_dep 1.5.0
Using listen 3.1.5
Using method_source 1.0.0
Fetching mysql2 0.5.3


Your user account isn't allowed to install to the system RubyGems.
  You can cancel this installation and run:

      bundle install --path vendor/bundle

  to install the gems into ./vendor/bundle/, or you can enter your password
  and install the bundled gems to RubyGems using sudo.

  Password: 
Installing mysql2 0.5.3 with native extensions
Using puma 4.3.3
Using rack-proxy 0.6.5
Using thor 1.0.1
Using railties 6.0.2.2
Using sprockets 4.0.0
Using sprockets-rails 3.2.1
Using rails 6.0.2.2
Using rubyzip 2.3.0
Using sassc 2.3.0
Using tilt 2.0.10
Using sassc-rails 2.1.2
Using sass-rails 6.0.0
Using selenium-webdriver 3.142.7
Using spring 2.1.0
Using spring-watcher-listen 2.0.1
Using turbolinks-source 5.2.0
Using turbolinks 5.2.1
Using web-console 4.0.1
Using webdrivers 4.3.0
Using webpacker 4.2.2
Bundle complete! 17 Gemfile dependencies, 75 gems now installed.
Gems in the group production were not installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

Bundle complete!だって！
やったね！！！