sidecar unable to connect deviced timed out
I had this problem setting up a new computer. For me, what fixed it was going into Settings &gt; Accounts &gt; iCloud and checking KeyChain

It said I needed to update my account security and prompted me with a few security questions. This seems to have indirectly solved the problem; perhaps some minimum standard of security was not upheld and was causing quiet failure.

Unfortunately, I'm guessing that this won't solve everyone's problems as it seems to be a pretty generic message as mentioned above, just thought I'd add my solution to the pile for everyone who's going down the list and trying everything.