If you try to upload a GPS file and see an error message that your file is "Empty," it's incompatible with Strava. This error happens when there's no GPS data in the file you're trying to upload, primarily for two reasons:
Most often, if you get this error uploading a .TCX file, it's because your Garmin has overwritten an older activity with a more recent one. If you've already uploaded your activities to Garmin Connect or Garmin Training Center, you should be able to export a .TCX file from there instead, as long as the original activity includes your actual mapped route rather than just time and distance totals.
If you get this error when uploading a .FIT file, it's likely due to corruption in the file itself — it's often extremely small, sometimes just 72 bytes.
Unfortunately, in both cases the files can't be repaired to make them compatible with Strava. Your best options are to ask a friend you recorded the activity with to share their original file for you to upload, invite you to the activity, or create a manual activity. If you have additional questions, please create a support ticket.