[Alan Anderson]

That error usually points to an attempt to use a Set on a motor that you didn't successfully open.

Just for completeness, check your Talon SRX device IDs again. Better yet, have someone else check them. Then inspect the code to make sure you're doing a Motor Open followed by a Refnum Set (in Begin.vi) with the same name you later retrieve with Refnum Get. You'd get a different error if the spelling was off, so I suspect the problem is either with the Open function or with the way you're providing the Refnum to the Set function.