Likely there is a problem with the file. Attach it 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.
