when you add a floor you need to specify the floor number
possibly you entered an index that is all ready in use on another floor in this building.
floor-0 is the ground floor
floor-1 is the first level not the floor that in some countries is refered to as first floor
a floor number that is in use is still visible in the drop-down list.
-> check other floors
also check properties of the building with regards to number of floors and number of basements.
if you reached the max number of floors, you cannot add another (number not available in the drop-down list)