Here is another important Data Point.
Originally, when booking the LifeMiles award on UA, the Saved Passenger in Lifemiles automatically populated the reservation with the Avianca Lifemiles frequent flier number and information. This was not editable on United.com. Subsequently, when trying to add bags, it wanted to charge. Even after I added this to My Trips, it still did not help.
I went back to Lifemiles.com and was unable to change this.
I then logged out of Lifemiles.com and went to Avianca.com and found the reservation while logged out (via confirmation number alone). I then changed the FF # from Lifemiles to United MileagePlus as Dan suggested.
Then I logged back into United.com and voila, I have free bags associated with the reservation.
Apparently, you need to make sure that the reservation is connected to your UA MP account and not connected to any other account.
(Add-purchase bags is not enough, you need to proactively disassociate other accounts and connect your UA account).