As I only wanted to test Adobe Muse first, I downloaded the trial.
Now after my trial has expired I can only "buy" it although it should be part of my Adobe CC subscription which includes Muse.
I tried to re-download, restart, and uninstall Muse but it only gives me the option to "buy".
Log out of your Cloud account... Restart your computer... Log in to your paid Cloud account
A chat session where an agent may remotely look inside your computer may help
Creative Cloud chat support (all Creative Cloud customer service issues)