I don't think you are missing anything, it will continue to be orange until you complete that row. A row of tiles needs to be connected to wall at both ends, once you finish that row and connect it to the wall on the other end it will work. Like the first row, it is a complete row touching walls on both ending points. Unless you are saying it won't let you create a second floor at all even with a complete row... You need to make sure there is wall touching all the borders of the flooring.