Unable to sync music to Apple Watch SE2

Recently upgraded to iPhone Air from 13 mini and had to re-pair my Apple Watch SE2.

Both updated to the latest OS.

So I had to re-sync music from my phone to the Apple Watch.

I like just having the watch with me when I go to the gym or go on runs.

But it's not working.

When I add my Albums and Playlists from the Watch App, it just shows that spinning animation for a few seconds and stops - but nothing is added on the watch - even if it's charging and connected to the phone and to the wifi.

I'm not subscribed to Apple Music. Most of my library is offline.

Never had a problem with syncing before with the iPhone 13mini and the iPhone SE3 before that.

I don't undestand what's wrong this time.