Not sure Andy if this is one of your reasons - but if you've done editing on your recorded tracks, perhaps they need to be consolidated - too many edit points and I've had plugins refuse and I've seen that message.

So Select your track, go to the tracks menu, find and click on consolidate. Then try what you want to do.

If you already know about consolidate, then my apologies if I've insulted you.

Hope it works. Cheers - Ian

Ian
My "Original Tunes" Site
My gene pool needs more chlorine.