If you look at your Level 1 floor plan, its associated level is Ground Floor, but your walls are 0.6m higher than Ground Floor. Also, some of your walls are not room bounding.
First, select all your walls and check the Room Bounding.
After that you will have 4 options to make it work:
1. Go to an Elevation or Section, select the Ground Level, type 0.6 in Computation Height. After that, you should be able to add room in your Level 1 floor plan.
2. Select all the walls that 0.6m from the Ground Level, lower them to touch the Ground Level.
3. In your Level 1 floor plan, use Room Separator to draw over your walls.
4. Create a new floor plan with the Associated Level to PLINTH FLOOR LVL.
Let me know if you still have questions.