@Suzanne726
It is allways difficult to help the other party, as you can not see what they are doing or what is on their screens. If they can not fulfill the booking process, they better contact Airbnb.
If the "special offer" option is available, you can sent the booking to them by yourself.
If the guests states "dates won’t show up on the calendar at all", you can easily check yourself if the desired dates are available to book. Or maybe the booking request did come in already and now waiting for your acceptance ?
Best regards,
Emiel