@Lillian94 This a community discussion board for hosts and guests, not Airbnb customer service.
But that message is real. It indicates that there is something amiss with this booking or guest. Either their payment didn't go through, or it was found to be a stolen credit card, or the guest has been reported to Airbnb for bad behavior or criminal activity.
Is this guest in residence at your place now? Airbnb will have sent the guest a cancellation notice, telling them to leave. Which they need to do. Are you alone there? If the guest hasn't arrived yet, don't let them in. The guest isn't necessarily a danger to you, it may just be a matter if you not getting paid for the booking, but you don't know why Airbnb has flagged this booking and they won't give you details.
The end of that message says to reply directly to it by email. You should do that if you need help to make sure the guest doesn't stay.