Hello @Fran39,
For stays of fewer than 28 nights guests always pay in advance. Airbnb charges their payment card immediately when the reservation is confirmed. Airbnb then pays the host a day or so after the guest checks in.
If the stay is for 28 nights or more then Airbnb charges, on confirmation of the reservation, for the first month and then monthly, or part thereof, afterwards.
The article below has more information on when guests are charged.
https://www.airbnb.ie/help/article/92/when-am-i-charged-for-a-reservation
If it's a reservation for more than 28 nights and the guest wants to pay in full up front then, as all payment issues are a matter for the guest to sort out with Airbnb, they must contact Airbnb support.
Steve.