Mutators can enable/disable building
depending on how you island was built you have to ensure the floor piece your wanting to place a build on is adhering to the build grid
Think of the space as a 1x1x1 square grid of thousands of squares. If the floor piece doesnt align with this grid then the build cannot anchor to the grid and itll show the build piece on the screen but itll no doubt be red. There are methods to help changing this if thats the case - I would advice placing a grid plane it should be in the fortnite folder as this by default allows building in any height location i believe and test if you can build on it within the same island thatll confirm if its the build grid issue. Please test and report back 