I Cannot select the checkbox next to Photos in my iCloud Account on My MacBook Air

Im trying to sync my photos across my devices on the icloud, but when i go into system preferences and into my icloud settings i cannot check the box enabling Photo's- it just asks me to enter my password to enable the change, but the box remains unchecked.

I have tried 'repairing' the photo library, but that doesnt appear to affect the preferences.

Can anyone help please?

Thanks in advance.