Howard Miller

You MIGHT want to ask yourself if Moodle requires MySQL 8.0 or newer, what might happen if you "trick" it into using an older version...

Generally speaking, this is VERY bad advice.

This is line 900 in moodle_database.php...

https://github.com/moodle/moodle/blob/52c0da7c647bd6ba8c5f61882d88959821a1fb41/lib/dml/moodle_database.php#L900

It's looking for ddlib.php (not ddlib.nhp). Go and have a look at that line in that file on your install. Is it the same (correct)?

When you use an optional plugin, make sure it supports the version of Moodle that you want to use. If it doesn't it might still work but it's down to you to test it. You should not be surprised if it doesn't work. Things change between versions of Moodle.

I don't know anything about this plugin - but, in general, using a plugin that hasn't been updated for 5 point versions is going to catch up with you eventually.

This is the availability/condition directory in 4.5...

https://github.com/moodle/moodle/tree/MOODLE_405_STABLE/availability/condition

There's NO 'paypal' directory. So where did that directory come from in your installation?

I assume you must have added it. The only plugin I could find is - https://moodle.org/plugins/availability_paypal - and that tops out at version 4.0. Not even 4.3, never mind 4.5.