@Zainab44
Sorry this happened and frustrating for sure.
I agree with @Helen3 in that you should have contacted Airbnb and had them cancel the stay as it was a 3rd party booking and unauthorized extra guests. Once you allowed the booking and extra guests to stay, Airbnb may then say Aircover doesn't apply, as you knowingly allowed a 3rd party to stay and also allowed an unregistered guest. Just a guess on my part though since they didn't give you a reason for denying the claim.
This is the problem Hosts face when allowing a 3rd party booking to proceed. Airbnb will say it's up to you if you want to allow the 3rd party booking, but it is in violation of their policies and therefore you may not be covered if something like this occurs.
Third Party Reservations
https://www.airbnb.com/help/article/427