Watch, mobile app and website all sync fine via wifi, but with the watch (forerunner 255 music) plugged directly into a laptop (macbook air) I get an error message. No specifics, just a red banner saying there's an error. Watch and Garmin Express are all running the latest updates. I've tried everything I can think of short of rebooting the computer, including reinstalling Express.
Welcome to the club. You'll slowly find that most of us have that error. Reinstalling won't work. Rebuilding your computer won't work. There is a corrupt file on your watch. Which one? 
I have tried deleting all my activities from the watch and deleted the Metrics folder. I have been through all sorts of measures. None work.
A full hard reset of your watch DOES work, but it's only a matter of time before the issue returns. It's not a faulty watch either. I had this on the Fenix 3, Fenix 5, Fenix 7X, Epix Pro 51mm, and Edge 530 and Edge 1030.
Garmin seems unable to fix it and their developers don't even help us by naming the 'invalid file'. It hasn't been fixed for over 6 years, so don't expect a fix in the next 6 either.
Sorry, just giving you the facts...
I'm somewhat confident - to within ±0.01 of 100% that I've repeatedly unplugged and plugged my various devices over the past 6 years.
Garmin support have been contacted in the past and on more than one occasion. They recommended the hard reset. 
Just try and avoid Express is my recommendation.
