Have you tried to re-sign-in via the Creative Cloud app. So quit Lr, re-sign-in via CC app and restart Lr.
Let me know your results.
I am having this problem. I have signed out of CC, signed back in, and restarted LR. When I click on the little turn-down arrow at the top, it says "Sync with Lightroom Mobile … Sign In" - but when I click on that, nothing at all happens. I am signed in to CC fine.
I was able to get it to work by signing out of the CC account from LR's help menu, rebooting, restarting CC and then LR a few times.