Magento Warning display while we access the website or try to run the composer install command.
Actual Warning Message:
Warning: require(/var/www/html/vendor/composer/../phpstan/phpstan/bootstrap.php): failed to open stream: No such file or directory in /var/www/html/vendor/composer/autoload_real.php on line 73.
You can try to run the command,
If still not resolved, You need to do some manual steps to fix the issue,
Just remove the vendor/composer folder from your Magento root path,
rm -rf vendor/composer
Now Run the composer install command from the root of your system by CLI tools,
Wait upto finish the installation process.
Running the setup upgrade command and clear the cache will resolve the issue.
php bin/magento setup:up php bin/magento cache:flush