Unable to save a game

I'm a new player, so sorry for this really silly question.

I've been playing for a few days now but am unable to save my single player game. I have to rely on the auto save and reload that. This means I can't just leave the game when I like as I have to wait for an auto save to have occurred first.

The save button on the save screen is grayed out (disabled) when I see it, and it won't click.

What am I missing, why?

Thanks

Matt.