Hello @David830,
Presuming that today is not blocked on your calendar the problem might be due to your "prepartaion time" setting.
It could be that this is set to "Block 1 night before and after each reservation".
To checkk this and change the setting go to your calendar.
Click on "availability settings" and then on "reservation preferences".
Then look at the "Preparation time" box.
If that's set to something other than "none", click on it and set it to "none".
You can change it back after the guest's change is accepted if you want to have a clear day between bookings in the future.
Steve.