Play Game Services Client Server Skeleton Sample - ERROR: SIGN_IN_REQUIRED

[Star-Frog]

Hi, I followed the instructions: [https://github.com/playgameservices/clientserverskeleton]

My goal is just server-side token confirmation (so I know if the client is the "real" one)

But I keep getting this error message on the GUI, after selecting the Play Games account (popup): DEVELOPER_ERROR

I've done everything according to the instructions and experimented for many hours, but I just can't get it right.

Does anyone know the problem or the cause?

I also noticed that certain parts of the model appear to be 32-bit, how can this be "fixed"?
Are the dependencies outdated, don't you need play-services-v2?

dependencies {
     implementation "com.google.android.gms:play-services-auth:20.6.0"
     implementation "com.google.android.gms:play-services-games:20.6.0"
     implementation 'com.android.volley:volley:1.2.1'
     implementation 'com.google.code.gson:gson:2.8.0'
}