When the option to change my availability has been locked, screen should state that.
When MS introduced greyed out menu options it drove us all mad, and still does. "You can look but you can't touch." Horizon goes yet another step and doesn't even show the availability option greyed out when Availability has been locked by the Scheduler.
This functionality already exists. A day that is locked for editing displays a locked message. A copy of this interface can bee seen here: https://horizonwebref.uservoice.com/knowledgebase/articles/1117399