Add site button missign

It seems Registration is disabled on the site.

As the super-admin, you can always create new sites from the Network Admin screen (My Sites => Network Admin => Sites) irrespective of the registration setting.

If you want to be able to create new subsites from any individual subsite’s admin dashboard, you need to enable site registration (at a minimum “Logged in users may register new sites” in the Network Settings page).

Good luck!