If a day on your Availability Calendar is showing that it is Locked for Editing, then your assignor has the "Availability Locking" feature turned on. If this is the case, then you are not permitted to change your availability for that day at this time. Only your assignor can change your availability at that point. The "Availability Locking" feature is an optional feature that your assignor can turn on or off as they desire so you would need to contact them to inquire about their specific assigning practices.
You can read more about the Availability Locking feature at this support article.