Some code is trying to use the each function which was removed in PHP 8.0.
I checked the source code for the 0.5.6 release as well as the source code for the current development builds and neither of them use this function, which means the issue is either coming from something that you added to FOSSBilling or a dependency FOSSBilling uses (which seems pretty unlikely, truthfully).
You'll have to find out where the error is coming from. If it's in a dependency we use, we need to report the issue upstream to them. If it's in something you added to FOSSBilling, you will either need to remove it or update the code to be compatible.
To find where the error is occurring, edit your config.php file and set debug to true.
Make sure it's this one and not the one under twig.
If you do it right, the error page will change to display the full stacktrace which includes where the error is occurring.
