Dagmar,
I was just told by Airbnb that there is a glitch in the system right now with Airbnb that doesn't allow for alot of things including receiving a booking from my guests. If you notice, I just wrote to everyone on this message board letting them know that the glitch is affecting a few things so maybe just maybe this is happening ot you as well.
May I ask - are you actually going to "Manage Listings" to do this? Do you see the area in your account settings where you can turn it ON or OFF?
If so, then you are doing it correctly, and its just not accepting it. All I can say is try later tonight or tomorrow and see if the Glitch is finally resolved. Good luck.
If you feel that me or another host have helped you, feel free to click on the "thumbs up" button at the end of any post. Thank you so much.
Aloha, Momi
Great way to contact Airbnb or via Twitter at AirbnbHelp / Facebook