Update: managed to get it working.

How did you fix it?

I had to use a system of "hostname\username" and the password for that account.

So if my target PC was named Calvin and my account was Hobbes, I would type in "Calvin\Hobbes" as my RDP username, and the password would be whatever the password was for the account called Hobbes.

Apparently RDP doesn't like Microsoft accounts.

Weird, but still, I had to just be sure. Might be helpful to know in the future? Who knows, but thanks for telling me. I appreciate it

For the record, my PC is not named Calvin and my account is not called Hobbes. Nice try