Delete black floor/open ceiling line

Thanks Maulahaz, I've edited differently, a new wall, the hole ceiling is gone.....

Probably it can be done with this steps: 1. Go to Plane view 2. Select that Black Floor, or Floor next by 3. Select EDIT FLOOR from the Menu (right side) 4. Display will change into EDIT FLOOR View 5. Delete that Black Floor Hope it works. (Attached is just for ref only)