Hi @Heloise10 I hope you are well.
There could be any number of resons why the potential guests payment can't be collected by Airbnb- there might be an issue with their payment system, there could be an issue with the customers card or funds, it would be impossible to say.
It really is up to the guest to make direct contact with Airbnb customer services themselves to remedy the situation, as hosts we have nothing to do with the transaction between guests and Airbnb.
Paul 🙂