Choose the surface , after Create flow region

I want to simulated the external cfd for an office room with some people that one of them Coughing, and the air flow come from the window , from a cuboidal enclosure . when I set all of that, gives me an error, about that you have more than one solid . After that I boolean union all of the boxes and import one solid (from Rhino) and the error has been solved but now I can not choose the surface for Coughing or in set boundary condition , my layers does not come and I can not define tables or people or any thing else.
please help me
what is wrong ? what I should be do?
How can I choose the surface for Coughing , after Create flow region?

and I have another question , in one of the page I saw the simulated CO2 concentration (Using CFD to Simulate Ventilation Equipment | Blog | SimScale) but Ido not Understand , how do it
please help me

Hi there!

I can see that you have a successful simulation. Can you please confirm that you were able to solve the issue?

About the CO2 concentration model, it uses passive scalars. Please refer to the following documentation to learn how to use it:φ-flux-and-t1-stand-for/