selected images are not compatible with each other for merging

Ive gone through a senario ive done hundreds of times merging images but this time i get "selected images are not compatible with each other for merging" this is the first time ive tried to merge images that were taken with a manual lens, but i dont see why that should be an issue, ive tried merging less images, Two instead of the five i want to, ive converted the CR2 images to DNG but still nothing

Any help gratefully accepted, Thanks in advance, Paul