All positions should have exactly one king for each side. No more than 8 pawns per side, no pawns on the first or eighth ranks, both kings can't be in check, and the player not on the move can't be in check. One side to move with no legal moves probably won't work, but not sure.
Pretty much anything else should probably work.
I got the Chess.com app barely two days ago, so I apologize if there is a glaring solution to this... I’ve been tinkering with the app just trying to get a feel for how it works and I’d discovered the custom positioning option when playing with a computer which seems like it could be very fun, however almost any variation I lay out gives me the message “invalid position” and will not let me play! The only time this does not happen is if I make a very minute change like flip the king and queen, make a power piece a pawn and vice versa, etc... I’m certain this is simply me not understanding some rules, so I guess I’m just asking if somebody could explain some of it to me. Thanks!