I get a script error when I run a script check in my game from the following line in VA#USTRN.BCS Should this be some other creature, or is there truly a missing resource?

That script you are talking about belongs to Ustrain cerature. It was created by Sikret and it automaticaly determines if IA is installed or not. If installed, then creatures from IA are summoned during the battle with Ustrain, and this is one of them. So the battle is harder. If not, then some original creatures from original game resources will be summoned... So it is not a "bug" or "fault", this is how it´s intended... If your script check is showing an error it is because you have not IA installed I guess.