@Kadek-Ayu0 First of all, I am sorry this has happened. As the booking was for 30 days, your strict cancellation policy would have been superseded by Airbnb’s own long term stay policy
https://www.google.co.uk/search?q=airbnb+cancellation+policy+long+term&ie=UTF-8&oe=UTF-8&hl=en-gb&cl...
It is also possible that Airbnb were unable to get payment from the guest or that they became aware of some other issues with them. All you can do is persevere in trying to find out what happened. However, at this point I would probably just let it go. Customer Support are notoriously difficult to deal with at times and it seems to me that when they have little or no experience on a particular issue they just keep passing the problem on to others. The positives in your case are that the guest did not ‘stay and not pay’ and that your calendar is open for other reservations.