ありがとうございます。

PHPのバージョンをあげて、

composer update symfony/flex --no-plugins --no-scripts
を行いましたが、以下のメッセージが返ってきました。
次に行うべきことがあれば宜しくお願い致します。

Loading composer repositories with package information
Updating dependencies (including require-dev)
Nothing to install or update
Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.
Package symfony/lts is abandoned, you should avoid using it. Use symfony/flex instead.
Generating optimized autoload files
Warning: Ambiguous class resolution, "Eccube\Entity\Product" was found in both "$baseDir . '/app/proxy/entity/Product.php" and "/src/Eccube/Entity/Product.php", the first will be used.


あと、無理やり
bin/console eccube:plugin:install --code=RecommendedProduct4
を行ったら、以下のエラーメッセージが返ってきました。

Error thrown while running command "eccube:plugin:install --code=RecommendedProduct4". Message: "/app/Plugin/RecommendedProduct4/composer.json not found." ["exception" => Eccube\Exception\PluginException { …},"command" => "eccube:plugin:install --code=RecommendedProduct4","message" => "/app/Plugin/RecommendedProduct4/composer.json not found."] ["file" => "/vendor/symfony/console/EventListener/ErrorListener.php","line" => 46,"class" => "Symfony\Component\Console\EventListener\ErrorListener","function" => "onConsoleError","uid" => "e35459d","user_id" => "N/A","session_id" => "N/A"]

In PluginService.php line 438:

/app/Plugin/RecommendedProduct4/composer.json not found.