Wondering what happens when you click the white "Try it now" button in your image.
Eg: Do you get to go to an option where you can click "already subscribed" ?
If you haven't already:
✽ You might want to try signing out, closing and re-opening the app, and signing back in.
✽ Would updating your Windows OS version help...
✽ Try going to music.apple.com.
Sign in with the Apple ID and password that you use with your Apple Music subscription.
Apple support has various "how to" articles online:
Eg: How to: Add and download music on Windows - Apple Support
If you use iCloud: Use iCloud Music Library on PC - Apple Support
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Online resource:
Apple Music User Guide for Windows - Apple Support
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Apple Music Support website: (info. / support / contact options etc.)
Music — Official Apple Support