apple music library on iphone

I turned off 'sync library' on my iPhone Apple music app. I turned it off because the app was loading my music files from my computer...and I don't want to lose the custom playlists. Now when I try to download an Apple album to my iphone to be streamed in my car only, I can't (I get a constant pop-up: "turn on sync library")

How do I create an Apple music library that is separate and cannot access my computer music files. I don't want to sync anything I find on the app to any of my other devices. I just want albums to stream to my car from the iPhone Apple music app.

Thank you.