Seeking Assistance with Google Play Console Closed Testing Setup Issues

An Android developer is encountering an error message '[Google sign-in failed: App not configured correctly (verify SHA-1/Client ID).]' when attempting to set up Closed Testing for their application in the Google Play Console. They have already tried creating a new release, checking the app bundle, verifying the testing track configuration, and confirming tester setup, but the issue persists. The developer is seeking guidance on what triggers this specific error and the correct method for resolving it, asking if others have experienced and overcome this problem.