Getting Failed when linking a device for mobile push notifications in flutter

Current scenario:

The client is linking a device when the user logins to the bot, it’s linking successfully for the first time.
And they are unlinking the device when user log outs.

But when the user logins again getting failed message while linking a device.

Need help in this case. Please help with what steps the client has to follow for mobile push notifications and how they have to configure from their end.