I can use may apple music using my family share plan, but when I try to add Apple Music to Sonos, I get the message that I am not a subscriber.

Based on the first part of your title, I'm assuming that at your computer or on your iPhone/iPad you can use Apple Music to play songs you've selected that you do not personally own. Is this the case?

As to the second half. Do you mean you cannot add Apple Music as one of the music sources for Sonos? Or do you mean you've added it but when you try to select some songs to play you get the error message.