I cannot manually upload TCX or GPX files to Garmin Connect--uploads fail

Please zip then insert the files into your post. We can take a look at it and see what is wrong.

Hi Volkan, Garmin Connect likes time stamps on any gpx type files it imports. Unfortunately Basecamp does not seem to be able to add time stamps. If I have created a file in Basecamp (e.g. to complete an activity which has a segment missing) I will export it as a gpx then open it in a gpx editor program such as the free version of Adze and edit the time stamps of the new segment to times that fit the activity then save as gpx. Garmin Connect will then import this gpx with no problem. Good luck. 

As noted, hard to tell without seeing the actual GPX file but it does sound like missing time stamps.  Another tool to fix this is available from Strava Tools that includes(among a bunch of useful tools) one to Add or Adjust Timestamps to a GPX/TCX file.  It's easy to use - pick a start time, provide an average speed or pace (you can even have it adjust based on elevation) and then run.  Results will be a GPX file simulating a run/walk/cycle whatever.  The site has a couple of other really useful tools regardless of whether you are using Strava or not.

Any help?

Also you can try to upload your GPX or TCX file to Komoot with "Import a GPS file" tool. Rename file in Komoot, set the type of activity and save as route. Think then it must be exported to Garmin Connect, of course, if GC and Komoot accounts are connected.