Finished activity and it didn't sync to Strava.
Just 1 activity? Did it sync to Garmin Connect? If not, try a manual upload of your activity to Garmin Connect - How Do I Manually Upload an Activity I Recorded to Garmin Connect?
If you get any message other than Import Complete, then there is a problem with the file. It is likely corrupt. Attach it to this thread. We can take a look to see if it is repairable.
Yes it synced in Garmin that's why I don't understand why it didn't sync in Strava as it normally does.
If it's just 1 activity, could be just a 1 off and there might be something about it that Strava will not accept. Did you try a manual upload to strava as well? Upload and Sync Your Activities | Strava