If you have double booked the guest @Shanique1 then if you can no longer host him you need to contact Airbnb urgently to cancel the booking so they can help the guest find alternative accommodation. (There are penalties for doing so as Airbnb takes host cancellations seriously)
How is it that you and your co-host aren’t using the same calendars? You need to link any calendars going forward so this doesn’t happen.
give Airbnb a call and text guest as well as message - many don’t have Airbnb accounts set up to receive messages