For an unknown reason, when I tried to build my Google App Engine endpoints, I get these errors in all of the API java files generated by Android Studio:
Err
Same thing happened to me this morning.
I resolved it by adding this in my backend project
appengine { endpoints { googleClientVersion = '1.23.0' } }
and updating this version in my app gradle file.
implementation('com.google.api-client:google-api-client-android:1.23.0')