Read this link- it may be an answer
In the 'Export' dialog window, the "Post Processing" panel "After Export" must not be blank.
Select 'Do Nothing' in this field and try export again.
Look at your Post Processing Action on the Export panel and change it from a blank thing to None. LR is trying to kick off a process at the end of the Export and it cannot start a blank command. If you set None then it won’t try to do a post-processing action.