Changing a reservation date as agreed with guest

I have agreed with an upcoming guest to change their booking dates. I have updated the calendar to make the day before available and then through their booking clicked on the change reservation dates button.

However it will not let me change the dates. The calendar for the change the dates does not show the date being available to select and I have tried to type it in instead but it won't go through to save.

What am I doing wrong?