This is an issue with the Google Play Games app, but there are a few steps you can take to try to solve the problem. 

Firstly, we tend to find that a fresh launch tends to fix things so try the following:

1) Close all the apps running on your device, starting with the game
2) Check to make sure you have ample storage on your device (and clear your device's cache too)
3) Turn your device off and on (restart tends to do the trick) 
4) Then launch the game before you launch anything else

If this does not work, try logging in to Play Games. If you see a "signing in is required prompt", there's something wrong with Play Games itself, in which case you should check if the Play Games app is up-to-date and check other games to see if you have the same problem. If you do, I recommend you Google the issue for a solution or escalate it to the Play Games team.