RH11: Error on Generate Responsive Output

I have created a screen layout (Standard), saved, and previewed it successfully.

I have selected Responsive HTML5 as my Primary Output.

However, when I click the Generate Primary Output button, I get;

Starting compilation...
Responsive HTML5 preprocessor 11.0.0.179

Clearing output folder...
Screen layouts used in this single-source layout are missing. Make sure that the required screen layouts are added in the project.

Internal error encountered,  Failed to generate Responsive HTML5.
Compilation complete.

What am I doing wrong?

Thank you!