Need help simulating wind flow through a house

Hello, after tinkering around for awhile, I have reached the simulation step.

However, I got the error message: “A setup with 37 regions is being used in a single-region analysis type. Please make sure that all regions except for one have an Advanced Concept assigned to them”

I found a forum post that raised a similar issue:

The first advice is to make a boolean union in the CAD software, which I have already done, and the second is to select ‘Add geometry operation’ and merge all solids into one.

I am not sure how to proceed after this step to merge all the solids together, can someone help me?