The answer is
“Tokei uriba wa rokkai desu.”
6th floor is not “Roku-kai”, but is “Rokkai”, which is an exception.
Other exceptions are 1st and 10th floor.
1st floor: Ikkai
2th floor: Ni-kai
3rd floor: San-kai
4th floor: Yon-kai
5th floor: Go-kai
6th floor: Rokkai
7th floor: Nana-kai
8th floor: Hachi-kai
9th floor: Kyuu-kai
10th floor: Jyukkai
