It looks like you had uploaded the same .mltbx as a different add-on and deleted that add-on. To prevent the users from uploading the same .mltbx multiple times, we check that the .mltbx does not exist in our system and if it does, we should the error message as you posted above.
I completely removed your deleted add-on with .mltbx from File Exchange. You should be all set to upload .mltbx with your existing add-on.
Let me know if you still encounter any issues.