@Nicoli36,. Good morning.
I'm a Host (on Cape Cod) not an Airbnb Representative. What was your Host's cancellation policy at the time of booking? (You should find it in the reservation confirmation email you received from Airbnb).
When you attempted to contact your Host did you send him/her a message via Airbnb message inbox? (Don't call or text the Host. Always communicate on the Airbnb platform so that all parties involved can see exactly what was discussed, and when).
I suggest going to your own Airbnb page, click on "TRIPS". Select this current reservation. Select "REQUEST TO CHANGE RESERVATION"...or similar wording.
Then adjust your ck-out date.
I believe the Host will receive a message from Airbnb asking the Host to accept your reservation change request. I hope it works out for you.
Good luck.