Make sure you got Firebase Service Account JSON for web panel and google-services.json for Android app from same Firebase account. You can get the Firebase service account JSON from same account you got your google-services.json and upload it in Settings→Features.
https://smsgateway.rbsoft.org/docs/downloading_firebase_service_account_json.htm
Or you can get the new google-services.json from same account where you got your Firebase service account JSON and create new customized APK.
https://smsgateway.rbsoft.org/docs/create_customized_android_application.htm
Or you can use general APK from below link and use it. General APK doesn't use Firebase, so you won't get this issue.
https://rbsoft.org/downloads/sms-gateway/sms-gateway.apk
If you are using web panel or android app version 9.4 or below then you can follow below guide to solve this issue.