You can try free online tools, such as FIT File Tools . Run it through the section remover without any adjustments, then try to upload to Garmin Connect Web. If still issues, attach your fit file to this thread. We can take a look to see if it is repairable.
Zip the file, then in a reply post Insert>>Image/video/file>>click Upload>>browse for file>>OK.
Or upload the fit file to a reputable file sharing service, such as OneDrive, Google Drive, Dropbox, make the file public, then paste a link to the file in your post.
