To use this app on your Apple Watch, the iPhone app for Music needs to be installed on (null)

After I paired my new phone to my Apple Watch SE, I can’t sync photos. But what I really want to know is what’s wrong with my Apple Music. When I open up the app on my watch it says “To use this app on your Apple Watch, the iPhone app for Music needs to be installed on (null). Please help

[Re-Titled by Moderator]