This has happened a few times where the activity from my forerunner 35 does not sync. I can see the activity on my watch just can't sync it to the Garmin Connect.
Try a manual upload of your activity to Garmin Connect



If you get any message other than Import Complete, then there is a problem with the file. In that case, please zip the file and attach it to this thread. We can look at it to see what might be wrong.
If you're on macOS: Steps to Manually Upload Activity Data to Your Garmin Connect Account click on directions for a Mac.
Hello! I was wondering if you would be able to fix this file again? I am have the same issue I was having a few months ago. B76A1306.zip
Here ya go - B76A1306-fixed.fit
Repaired using Fit File Repair Tool
Some suggestions to help minimize the occurrence of corrupt fit files:
Error Occurs When Manually Uploading Activity Data to Garmin Connect