Hi BondiDan,
Welcome to Apple Support Communities. We understand that you're having an issue with with enabling iCloud Photos on your MacBook Air. We'd like to help.
Have you ever used iCloud Photos on your MacBook Air before? Have you restarted your MacBook since you noticed you were not able to turn it on? If not, that quick step can help resolve issues like this. Give that a try following these steps and let us know how that goes:
Restart your Mac
- On your Mac, choose Apple menu
> Restart. - If you don’t want app windows that are open to reopen when your Mac restarts, deselect “Reopen windows when logging back in.”
If you still have issues, confirm that you're signed into the same Apple ID on both devices. You can do that on the iPhone at Settings > Tap your Name or on your Mac using the Apple menu > System Preferences or Settings > Tap your name.
Finally, check to see that you see the correct amount of iCloud Storage now that you've upgraded. From your Mac, go back to the Apple menu > Tap your name > iCloud. Is it showing your 2TB of storage?
Let us know and we'll continue to make a plan from here.
Cheers!