Make sure you have Game Center enabled in your capabilities section of exported Xcode project.
If you see attachments are not fetched from Game Center, make sure there are added to your draft version in iTunes Connect under Game Center section
Almost most of the reasons for the sign-in failed are covered under below three cases
Mismatched SHA (Ideally when using gradle you need to add 3 Linked apps with 3 different SHA - Debug, Release and new Google Play Signing mode when downloaded from play store market)
Saved Games is not on - When using Cloud Services
Drive API is not set to on in google cloud console
Incase you are not able to figure out what went wrong, please share us complete logcat log in development mode (with out any log filtering).