@Jeanie12 You are mixing up terms, so it's hard to be sure whether you got an Inquiry or a Request- Inquiry buttons are pre-approve, and Decline. Booking Request is Accept or decline. You start off saying it was an Inquiry, then at the end said the Accept/Decline button isn't working.
If it is an Inquiry, the CS rep was correct- you don't need to either pre-approve or decline an Inquiry. Just messaging back within 24 hours is all you need to do so as not to have your response rate dinged. Never mind the pesky reminders from Airbnb to pre-approve.
If it's a Booking Request, that's a different story- you do need to either accept or decline within 24 hours,
So I'd just make it clear to this guest that a booking like that won't work for your listing and wish her luck finding the perfect spot. Don't decline, as that affects your acceptance rate. If you want to decline an Inquiry, best just to message back letting the guest know why it won't work and hope they move on.