Reminder – Groove Music is partnering with Spotify to bring the world’s largest music streaming service to you. In 30 days, on December 31, 2017, the option to purchase more music will be discontinued.
Keep your current music collection intact by downloading to your devices any albums and tracks that you’ve already purchased. You can download your purchased music through the Groove Music App until December 31, 2017.

To download your music, open Groove, go to your music collection and select the Purchased filter. Right-click or press and hold your music files and select Download from the menu. We thank you for your support on this journey.

*Moving music and playlists from Groove Music Pass to Spotify will be available until at least January 31, 2018. We encourage Groove Music Pass customers to move any or all Groove Music Pass content to Spotify by this date.

Source: Email from Microsoft

See also: Microsoft to bring Spotify to Groove Music Pass customers - Windows 10 Forums