How to fix SERVICE_NOT_AVAILABLE error in firebase cloud messaging while getting token

I am trying to get fcm token and store it in cloud firestore:

val user = User(et_email_sign_up_activity.text.toString(), et_name_sign_up_activity.text.toString(),
                            et_mobile_sign_up_activity.text.toString(), "",
                        FirebaseAuth.getInstance().currentUser?.uid!!, tv_address.text!!.toString(), getToken())
                    Firebase().uploadUserSignIn(user, this)//uploads user to cloud firestore

getToken():

private fun getToken(): String{
        var token = ""
        FirebaseMessaging.getInstance().token.addOnSuccessListener {
            token = it
            Log.i("token", it)
        }.addOnFailureListener {
            val dialog = AlertDialog.Builder(this)
            dialog.setMessage(it.toString())
            dialog.show()
        }
        return token
    }

I always get this error whenever I try to execute my code in real android device:

E/FirebaseMessaging: Topic sync or token retrieval failed on hard
failure exceptions: java.util.concurrent.ExecutionException:
java.io.IOException: SERVICE_NOT_AVAILABLE. Won't retry the operation.

I have google play services running with proper internet connection

Few solutions I have tried:

1. Add internet permission in manifest:
2. Download google.json file from firebase after entering sha1 and sha256
3. Enable android device verification in google cloud.

Sometimes I don't get the error and fcm token is returned successfully returned but most of the times I get this error.

What is the error?