If you are using web panel v9.4 or above, make sure your Android app version is also v9.4 or above. From v9.4 it uses newer version of Firebase API, so if you did fresh installation of latest version it won't work with older APK. You can download the latest version of APK from here. This APK is generalized with Server URL field and don't use Firebase. If you want to use Firebase, you have to customize it using build server, as shown here.
If you are using web panel v9.3.1 or below, make sure your Android app version is also v9.3.1 or below. You can create APK for older version using build server as shown here. You don't have to get the "google-services.json" to create an older version of APK. It is recommended to upgrade web panel to latest version because Google is going to discontinue older version of Firebase API on June 2024. That can prevent an older version from working after that.