How do I purchase music in itunes when I've already downloaded it thru Apple Music Subscription?

You need to make sure you are accessing the iTunes store. The iTunes or Music app will only let you stream and download but the iTunes Store will let you purchase the music. I'm not sure about the Windows setup of iTunes/Music but on Music for Mac, there is an option on the left side bar to access the iTunes store where the music can be purchased.

This is strange. If I do the same thing on my Mac (right clicking on a song I downloaded from Apple music and click 'show in iTunes Store'), the song shows a price and not 'play' like it does for you. This may be a Windows related issue and I'm sorry to say that I have no experience with iTunes on Windows. But I'm sure somebody else in this forum will be able to shed some light on the issue. If everything fails you can always reach out to the official Apple support by phone or chat.

You will need to go to the iTunes store to purchase the song(s). The music/iTunes app will only let you stream.download the songs due to your Apple Music subscription.

You can also try right-clicking a song you want to purchase and select 'Show in iTunes Store' which will take you to the store to purchase it.

That looks like you already own the song. To verify this, right click the song in your library and select ‘get info’. Then select the file tab and see if it says ‘purchased’ under cloud status.

One more thing: what does it show when you click the arrow next to the 'play' button in the iTunes store?

You’re welcome. Wish I would’ve been able to solve it though. Definitely curious what the fix turns out to be.

Clumsy indeed. But if it works...