You need to select at least 2 photos by holding the Shift key or the Ctrl key (they get highlighted) to have the functions active.
of course I selected more than 2 photos yet in photo and merge, it refused
to work with greyed caption. young
What file format do the selected photos have?
In my LR6, I noticed that when I select photos for photo merge and immediately, before clicking anything else, select Photo merge, it works ok. But if I click even any selected photo (all photos still stay selected), the Photo merge menu option turns grey.