You need 25 players on your team, please adjust it before playing.
It’ll be patched Late Monday night
I had the same problem last night. I closed out the app and restarted and it was fine after that. When I restarted I also got RS rewards because I had just played online for the first time this season. Not sure if the two are related. Wouldn’t think so but you never know with the wonders of online gaming bugs.