Short answer: IM006 with "Token already exist." / "Token not found." appears when Make finishes an OAuth sign-in (Instagram for Business, Facebook, QuickBooks) but cannot save the token. Make's Help Center does not document this code. In every Make Community thread we found, deleting and recreating the connection did not fix it, and the answer was a ticket to Make support or a fix on Make's side. Check the few things you can check yourself, then open a ticket with the evidence below.
We could not reproduce IM006 for this guide. Everything here comes from Make's Instagram app docs and Make Community threads.