- Please reset Lightroom Preferences Recover catalog and images after resetting preferences
Then go to Preferences>>External editors in Lightroom and make sure External editors are setup correctly
Add them to the Editors list(if they do not show up)
- Also Make sure NIK Software is up to date
It may be some limitation I'm not aware of on my part. About two months ago, I created a custom Tone Curve that I've been applying to my images. None of the images to which I've applied that curve can be exported to an external editor. If I change the Tone Curve to one of the standard settings -- Linear, Medium, or Strong contrast -- I can work with the file in the external editor again ...
What happens when you select Edit in >>edit in other Application and then select say Elements.?
Also Backup your Lightroom catalog and reset Preferences See Recover catalog and images after resetting preferences