AWS S3のパッケージインストール時に、composer require league/flysystem-aws-s3-v3を叩くと、Your requirements could not be resolved to an installable set of packages.とエラーが出るときの解決策。

前提条件

・macOS
・Laravel6
・初学者

composer require league/flysystem-aws-s3-v3を叩いた結果

test@test laravel % composer require league/flysystem-aws-s3-v3

Using version ^2.0 for league/flysystem-aws-s3-v3
./composer.json has been updated
Running composer update league/flysystem-aws-s3-v3
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - league/flysystem-aws-s3-v3[2.0.0, ..., 2.x-dev] require league/flysystem ^2.0.0 -> found league/flysystem[2.0.0-alpha.1, ..., 2.x-dev] but the package is fixed to 1.1.3 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.
    - league/flysystem-aws-s3-v3[2.0.0-alpha.1, ..., 2.0.0-alpha.2] require league/flysystem 2.0.0-alpha.1 -> found league/flysystem[2.0.0-alpha.1] but the package is fixed to 1.1.3 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.
    - league/flysystem-aws-s3-v3[2.0.0-alpha.4, ..., 2.0.0-beta.1] require league/flysystem 2.0.0-alpha.3 -> found league/flysystem[2.0.0-alpha.3] but the package is fixed to 1.1.3 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.
    - league/flysystem-aws-s3-v3[2.0.0-beta.2, ..., 2.0.0-beta.3] require league/flysystem ^2.0.0-beta.1 -> found league/flysystem[2.0.0-beta.1, ..., 2.x-dev] but the package is fixed to 1.1.3 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.
    - league/flysystem-aws-s3-v3 2.0.0-RC1 requires league/flysystem ^2.0.0-RC1 -> found league/flysystem[2.0.0-RC1, ..., 2.x-dev] but the package is fixed to 1.1.3 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.
    - Root composer.json requires league/flysystem-aws-s3-v3 ^2.0 -> satisfiable by league/flysystem-aws-s3-v3[2.0.0-alpha.1, ..., 2.x-dev].

Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.

Installation failed, reverting ./composer.json and ./composer.lock to their original content. 

解決方法

バージョンがあってないよ〜。と言われている気がしたので、

test@test laravel % composer require league/flysystem-aws-s3-v3 ^1.0

バージョン指定を入れることで解決できました。

参考記事