Make sure you start with two or more photos of a subject (the same subject) that have been shot using different exposure settings.
Lightroom will show an error if you try to merge photos of different sizes, focal lengths, etc.
im having the same issue here ...
The answer appears to be in the reply above yours. Is that not the issue?
Ok worked thank you!
I was having this issue with 4 HDR photos that I had already merged into an HDR with Photomatix.
I was trying to use Camera Raw to merge them into a Panorama and test out the Boundary Warp feature.
I got this error too. I used Photoshopped to crop to a specific pixel height for all the photos. They were still slightly different widths.
Looks like for the Panorama merge it's only the heights that have to be the same.