Challenge Declined
I think that was pretty well phrased, actually. There is a bug in the Chess.com iPhone app (still there as of October 1, 2023) where trying to create a new game results in a “Challenge Declined” message. Even if you’re not actually challenging anyone, just trying to be paired for a 10-minute game (or 5-minute, or whatever; the bug is not limited to one type of game).
Unfortunately, unlike OP, restarting the app does not always work for me. I just have to wait it out. After a few hours, I can usually play again.