If it is try again later, there are 2 possibilities I've seen.
You lack in intelligence or charisma, you might need a minimum of 3. I'm not sure but try popping mentats and see what happens
Or the guard is attempting to walk somewhere (even if they're standing still) literally come back and try again later.
It might be something else, but I've seen similar responses from NPCs that were tied to these two. It can be a kind of generic catch all response.