Upgraded to IOS 14 this AM. I have an iPhone 11 and still run iTunes on my mid 2012 MBP 15/Sierra. When I tried to play music through AppleCarplay I got a message to enable it in AppleMusic which I do not have.
Thanks you are right. After I upgraded I went for ride in the car to try out apple play. I saw the red apple music icon and when I pressed on it. I assumed you needed to subscribe to apple music. Went I got home I went to iTunes and it had me approve something and now it works.I had a similar issue when I went to a version on 13 - what I had to do was open the Music app on my iPhone and it gave me a message to OK using it (or something similar - it was a while ago). Try opening the music app on your phone and see if it gives you a message