I contacted the apartment provider, who told me the apartment was available during the desired time frame. I also received an email from Airbnb stating that the apartment was reserved in my name until the following day, but then I discovered that someone else had already booked that apartment.
You probably submiited an inquiry, not a booking request.
When the inquiry is confirmed, you have the OPTION to book the appartment within 24 hours, but in the mean time somebody else also can book it. First come, first served.