After 45 minutes with web support trying to apply my Travelbank $ to a reservation, they finally figured out that the reason I cannot use it is because my reservation includes a codeshare. They figured out that Travelbank $ can only be used on itineraries that are UA metal all the way. I made some dummy domestic reservations with codeshares to test this and this appears to be true. Web support said they will be updating the website to indicate this limitation...