@Maria-Victoria5 This is an open forum of peers - Airbnb does not receive messages here.
But if it's not possible for you to accommodate this guest, you have no choice but to cancel the reservation. Be aware that there will be penalties for this action. To prevent it in the future, make sure to keep your calendar up to date and block off days that aren't available.
If you still need to contact Airbnb, hit the Back button and scroll up; the thread "How to contact Airbnb"is pinned to the top of this forum's topic list.