I'm messing around with a bunch of different animations, and it's working fine. Then it spits out this error: I'm not sure what it's looking for, but I've tried using many different stock baseposes and it still gives the same error. Anyone know how to fix this?

Yep, the problem is that you've keyed a root or eff. Go back through all of your roots and effs and make sure none of them have keys on any frames, including 0.

I'm using stock animations from BF1 and 2. I'm assuming that the problem is mixing them, yes?

If I recall correctly, you can mix the two. Try swapping to the Battlefront 1 basepose ( or 2 depending on which one you are currently using)

Okay, I found out what the problem was. I was using an attack animation from grievous without using his basepose, so it was looking for four arms when there were only two.