@Laura2592
I looked at your listing, it seems impossible to book not available dates.
Some dates are greyed out, because the minimum stay prevents to start a reservation on them.
They become available as soon a proper check-in date is choosen.
Maybe the guest is confusing those dates as being "blocked".
If preparation days are set, the amount of days after and before each reservation will be blocked.
Which are all shown in the calender.
If the guest is happy to shorten her stay with 1 night, then the "change" option will do the job.
If it is a technical error, Airbnb should cancel and provide full refund to the guest.
If the guests cancels, she should be sure receiving a full refund (cancellation screen will predict this).
Thanks for the warning,
Emiel