Android Firebase app distribution - Service credentials file does not exist. Please check the service credentials path and try again

Try to use $rootDir to get a path. For example if you pass you credentials file api-project-xxx-yyy.json to root directory than you can take it something like this:

    firebaseAppDistribution {
        ...
        serviceCredentialsFile="$rootDir/api-project-xxx-yyy.json"
    }