It could be just a simple ‘handshake’ fail. That’s something that fairly frequently crops up in the wake of OS updates. It’s the first thing I’d test…so the suggestions below are geared towards checking for & resolving that.
Did you update software for both devices or only your phone? If the latter, then my first suggestion would be to check for watchOS update.
Then just give both devices a hard restart. That’s suggestion number two.
Hopefully that helps! Usually handshake issues resolve really easily. Sometimes they’re indicative of a software bug and they get patched in the next OS iteration. In any case, if your issue prsists, you can definitely ping us back for additional troubleshooting ideas. Or reach out to apple support.
Sidenote: Cycling power on your devices(s) can effectively nullify/wipe some malware types (including a lot of the really insidious ones), so it’s a good idea to do this after security updates anyways.