Hello @Stuart-And-Joanne0,
The "Send special offer" facility is available only on reservation inquiries, not on reservation requests.
Airbnb's reasoning for this, they say, is that someone sending an inquiry has not yes decided on the booking so may be open to a special offer. Someone sending a reservation request has made the decision to book, is requesting to book so must be happy with the price.
Reservation Inquiry or Request What's the Difference?
If you haven't accepted the reservation you can message the guest asking them to cancel their request and submit a reservation inquiry instead. You will then get the opportunity to send them a special offer. They haven't yet paid so there's no problem for them to cancel the request.
Alternatively you could message the guest saying that you'll accept the reservation but will later send a reservation change that they must accept for it to take effect. For how to do this see the "Confirmed reservations" section at the the end of this article How do I send a guest a special offer?
Steve.