Not sure how many times I've answered this, certainly lots:
There's a minor bug with LR4 that seems to affect many users, basically when you first export the Post Processing/Export Action field at the bottom of the Export dialog is blank. This is invalid, and causes the error that you are getting. So to get rid of the error simply eneter a valid option in that field (e.g. "Do Nothing" is fine) and the problem goes away.