Hi, I managed to get the file downloaded from the garmin to my computer in a convoluted way.
You need to plug the garmin into the computer
Then click on the device in finder (mac) / file explorer (windows)
Click through to the route files in the device
Download or drag the route into your files or desktop
Open garmin connect or strava and find the 'upload' ride section
You might need to update the file name to .tcx before the next step
Then drag the file from your computer desktop / files (wherever you saved it) into Garmin connect / strava
This worked for me. Good luck!
I had this happen to me but only once or twice. No solution per say but I recovered the ride because the FIT file was there on the Edge. I connected it to the computer, recovered that FIT file, and then uploaded manually to Connect (web) and Strava. Never found out why that particular ride would not upload. And it is working fine now. So see if you can recover it that way if it matters enough to you.