One of the party of 20 had Covid, @Nicola2365 ? If that's the case, what happens is that they get reimbursed. It's the last holdout of the old "extenuating circumstances".
I feel for you. It's hard to take that hit. By its very nature it is so close to arrival that rebooking the listing is nearly impossible. And a party of 20 is bound to have someone in it with a positive test, at least where I am, with cases climbing like mad. What is it like where you are?
I have seen it written here that even if they had travel insurance, their insurer would be aware of Airbnb's policy, and so it would use that (by "that", I mean you, the host) first.
What a lovely barn you have! Sorry, I know that doesn't help.