Hello @Benno6,
In the past, I've sometimes encountered buttons not working when using the app on my Android phone. I found that I was able to get the job done when I logged on to the website using my laptop.
You could try a laptop if it doesn't work on the app or vice versa.
You could aslo contact Airbnb support and get them to decline it for you.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.