If nobody has addressed 'error 106' for you yet, I can add that I see it all the time. For my searches it's pretty frequent when a Delta segment would logically be involved - as it would be for your travel DL hub-ATL to DL hub-SLC. (But I sometimes get it with NW segments too.) You can try phoning web support but be prepared to pay the phone award ticketing fee if they can't resolve this so you can book online.