pumaが原因でRailsがインストールできないんだなこれが。
sqliteがなくてインストールできないのはbrewからsqliteを入れれば解決するんだけど

macOS Catalina 10.15.6
ruby 2.6.5
gem 3.0.3
puma 4.3.5

似たような質問にxCodeがインストールされていないのでは？なんて回答もあったけど
手元ではそれで解決せず。

エラーメッセージ（クリックで展開）

gem install puma -v '4.3.5' --source 'https://rubygems.org/'
Building native extensions. This could take a while...
ERROR:  Error installing puma:
	ERROR: Failed to build gem native extension.

    current directory: /Users/rin.nakashima/.anyenv/envs/rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.5/ext/puma_http11
/Users/rin.nakashima/.anyenv/envs/rbenv/versions/2.6.5/bin/ruby -I /Users/rin.nakashima/.anyenv/envs/rbenv/versions/2.6.5/lib/ruby/2.6.0 -r ./siteconf20200810-80454-doo97b.rb extconf.rb
checking for BIO_read() in -lcrypto... yes
checking for SSL_CTX_new() in -lssl... yes
checking for openssl/bio.h... yes
checking for DTLS_method() in openssl/ssl.h... yes
checking for TLS_server_method() in openssl/ssl.h... yes
checking for SSL_CTX_set_min_proto_version in openssl/ssl.h... yes
creating Makefile

current directory: /Users/rin.nakashima/.anyenv/envs/rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.5/ext/puma_http11
make "DESTDIR=" clean

current directory: /Users/rin.nakashima/.anyenv/envs/rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.5/ext/puma_http11
make "DESTDIR="
compiling http11_parser.c
ext/puma_http11/http11_parser.c:44:18: warning: unused variable 'puma_parser_en_main' [-Wunused-const-variable]
static const int puma_parser_en_main = 1;
                 ^
1 warning generated.
compiling io_buffer.c
compiling mini_ssl.c
mini_ssl.c:145:7: warning: unused variable 'min' [-Wunused-variable]
  int min, ssl_options;
      ^
mini_ssl.c:299:40: warning: function 'raise_error' could be declared with attribute 'noreturn' [-Wmissing-noreturn]
void raise_error(SSL* ssl, int result) {
                                       ^
2 warnings generated.
compiling puma_http11.c
puma_http11.c:203:22: error: implicitly declaring library function 'isspace' with type 'int (int)' [-Werror,-Wimplicit-function-declaration]
  while (vlen > 0 && isspace(value[vlen - 1])) vlen--;
                     ^
puma_http11.c:203:22: note: include the header <ctype.h> or explicitly provide a declaration for 'isspace'
1 error generated.
make: *** [puma_http11.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/rin.nakashima/.anyenv/envs/rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.5 for inspection.
Results logged to /Users/rin.nakashima/.anyenv/envs/rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-19/2.6.0/puma-4.3.5/gem_make.out

ググってたらissueを見つけまして、Mac側のデフォルトパラメータが変わってgccでのコンパイルがうまくできなくなったらしい。
https://github.com/puma/puma/issues/2304
ワークアラウンドとして以下のように対処せよとのこと

gem install puma:4.3.5 -- --with-cflags="-Wno-error=implicit-function-declaration