If you already had 10 devices authorized on your Google Music than no new device will be added as Google puts limit to device deauthorization. you can add upto 10 devices. Previously there was no limit on what you could deauthorize. Once you hit the 10 device limit, you can only deauthorize 4 devices per year. According to XDA-Developers, some users have had success e-mailing Google support and having them deauthorize older devices.
There’s nothing official from Google side but users over at XDA are reporting about it.