Clean URL link for page without number

This is because your site already has a post or page (6 in your case) with the slug “photo.”

They might be in the trash, or pending as drafts, or whatever … but WordPress automatically appends number to prevent name conflicts.

So go through your site, and figure out which other pages are trying to use the “photo” slug and change their slug to something else. Then you can re-edit your other page to change its slug to just photo rather than photo-7.