@Kevin1989 if you have been in contact with someone positive and need to test, I believe you need to isolate, take a second test etc. I cannot remember the exact rules, but a German friend of ours needed to isolate in a situation like yours. You cannot possibly travel and isolate at the same time ?
You need to get a certificate from the appropriate body and present to airbnb CS if you would prefer a full refund. A partial refund would probably be possible if you cancel, depending on the host's cancellation policy.
If you test negative, you could travel after the isolation period I suppose ? Keep your host informed of the situation, perhaps you can come to a solution that would benefit both host and guest ?