As spotted by Thurrott, the music store on Google Play is no longer available. You cannot use it to buy music to keep.
The reason this is happening is that Google wants everyone to move over to YouTube Music, its new streaming service. You can easily transfer your music to the new platform by visiting the YouTube Music Transfer page.
Google had announced that this was happening. In fact, he gave the deadline end of August, so the extra two weeks is a bonus.
To export your previously purchased music, wishlist and reviews, go to Google Takeout. You can also sign in to your Google Play Music account if you want to delete your library and recommendation history.
If you decide to do nothing, Google says:
With this news, Google no longer sells music anywhere. The company takes a streaming-only approach.
If you still love owning your music, don't worry. There are many other places where you can buy music online, including Amazon and the iTunes Store. 
