A client reported issues with a 'readiness check' whereby the interface reported the incorrect memory_limit.
The phpinfo(); output shows whats set in the .user.ini, such as...
memory_limit 1024M
However, the readiness check was reporting the following...
Your current PHP memory limit is 512M. Magento 2 requires it to be set to 756M or more. As a user with root privileges, edit your php.ini file to increase memory_limit. (The command php --ini tells you where it is located.) After that, restart your web server and try again.
Further investigation revealed that the PHP variables being tested against were those applicable to the CLI version of PHP.
The solution is to also set the version of PHP for the CLI (in most cases, through the PHP Selector in cPanel) and set the variables through the same interface to 756M or above.
Then, the caches need to be cleared in Magento before re-attempting...
php /home/$USER/public_html/bin/magento cache:flush