On Raspbian (buster) I'm using, I was able to resolve the following error by upgrading PHP from 7.4 to 8.0: Configuration error: MPM not loaded.
When trying to enable mpm_prefork, it shows that the module does not exist: Error: Module mpm_prefork does not exist!
I did install libapache2-mod-php: apt-get install libapache2-mod-php
I have no clue, hope I can get help!
The installation command is sudo apt-get install apache2-mpm-prefork.
However, make sure you know what you are doing, what you have installed, etc.
Some configurations disable mpm_prefork and use mpm_event. To understand, look at your Apache configuration, which modules are enabled and configured, your application's requirements, your PHP vs. PHP-FPM configuration, and more.
It is also possible that you have both PHP 7.4 and PHP 8.0 configured at the same time, which may break your Apache configuration.
In the worst case scenario, you may need to completely uninstall PHP 7.4 and PHP 8.0, then install PHP 8.0.
Note: PHP 8.2 is the current version. Please use version 8.2 unless you have special requirements for 8.0.