Just in case you struggle with this like I did, the instructions in the video tell you to go to ‘Windows’ then ‘Mode’ to be able to add a new floor. This behaviour has changed, at least for version 4.25 that I am using, it is still under the ‘Window’ menu, but as a separate item call ‘Place Actor’.
I didn’t spot this and it took me a while to realise that was the new way of doing it. The whole Mode Panel appears to have changed considerably.
Hope that helps somebody