- Go to Lightroom > Preferences.
- Click on the General tab
- Uncheck “Show ‘Add Photos’ Screen”Restart Lightroom
- Also please Go to the Preferences > Performance Tab, and uncheck "Use Graphics Processor "
- Restart Lightroom and see if it works.
Thanks for the attempt Tanuj, but neither of those worked. Still can't get the export dialogue to pop up.
A number of people have encountered this bug, as described in this bug report in the official Adobe feedback forum: Lightroom: Can't access the Export dialogue. What works for at least some of them is to reset LR's preferences: Resetting the lightroom preferences file - updated - Lightroom Forums.
But before you reset the preferences, can you please save a copy of the .plist file? Adobe's Chief Customer Advocate has asked for a copy of the likely corrupted preferences so they can diagnose the problem. See his post here: Lightroom: Can't access the Export dialogue.
Please add your vote and opinion to the bug report, including whether resetting the preferences worked for you. That will help others and help Adobe, and you'll be notified of future updates to the problem.
Finally, I agree with Per Bernsten -- if you can't resolve this problem quickly, revert back to CC 2015.1.1 / 6.1.1.