I can’t execute this step, but when I look in log I have this problem
CLEditor is not packaged anymore with version 4.0 (since version 3.1 it has been replaced by TinyMCE)
The doc will be updated.
Anyway if you are using version 4.0’s response UI you should only look at the “Responsive UI pattern” section in this documentation, all other stuff refers to old shool patterns applicable to versions 3.x
The old way to write external objects should however still work with response UI but in an unefficient backward compatibility mode, see warning on the top of the doc:
Document has been updated to provide an example using TinyMCE instead of CLEditor.