What do I DO  Go to the Training: MySQL I challenge

What am I meant to do here? I've exhausted every resource I could find on SQL injection and the only thing I can get is a database error.

Maybe take your time and don't try to rush things

This is based on one of the simplest SQLI techniques, which is also the most widespread thing you should find when researching.
But of course, you can't just always simply copy paste a solution and hope that it works.

A database error is a good start, actually.
Check out the source code, and look where and how you can potentially influence the query executed.

Yeah, uh, problem with that, I'd already tried to use the source code to come up with my own input, and uh

Didn't work, gave the error again

I'd say the best way to understand what is happening is to run the code locally. That way you can debug it a bit (e.g. add some extra output) to see what is really happening.

Now, to really run the same code locally might be a bit involved, but setting up your own webserver with PHP and SQL will be useful for more than just this challenge. If you don't want to go that far now, you can get a long way by playing with online sandboxes that are available these days. Just make sure that for SQL the sandbox is using the same language variant as the challenge is (MySQL or MariaDB).

I'd break a quote for this: 'Good Luck's'!