challenge failed because either you or the other is playing

what does that mean? what even triggers this error? is it because I am on google chrome?

No, it means that you can't challenge someone who is already playing someone else.

This message comes up when I want to play the person I just played. We are no longer playing, but  we have timed up or one of us has won the game, I think. But people I have just played challenge me to another game. Why can I not make the challenge?

It's very possible your opponent has already started another game by then.

Thank you Martin, that is the only thing that makes sense, although they seem to start new games with the blink of an eye, (which is of course an exaggeration).

it means the one whom u are challenging is currently playing a game hope u understand thank you