@Kathy1088 There is nothing to contact Airbnb about- if a guest cancels within 24 hours of check-in, both parties get a notification to review. It isn't an error.
What you can do is send the guest a message or text saying, "Hi XX, Airbnb sends a notice to review if a guest cancels within 24 hours of check- in. I have just received one and you will, too. As you never checked in, we have nothing to base a review of each other on, so just letting you know that I plan to ignore the review prompt and assume you will, too."
And I don't understand a host not knowing how to contact Airbnb. There is a big pinned post on the first page of this Help section- " Contact Airbnb" that gives all the options. And a simple Google search calls up pages of results.