Hello @Margaret175,
On a computer you can go to your messages by clicking on "Messages" near your profile photo after you log in.
Then find the messages with the guest concerned and click on the text of the message.
You should then see a page with a box that syas "Pre-approval expired". In that box there should be a "Pre-approve" and a "Send special offer" button.
You can use either of these.
With the Android app you have the option of sending a special offer if you tap on the message and then on "Details" in the top right.
I don't have an iPhone so can't say what options the iOS app offers.
The guest may, of course, have already booked somewhere else for the dates.
Steve.