Hello!
In short, for Incompressible analysis, we need to have a single volume in our CAD, representing the flow region. Solid parts, such as buildings, should not be present: only the “negative” representation of them is necessary.
Therefore, to solve the issue, you can run a “Boolean”/subtract operation, removing all buildings from the flow region (solid65 in your case). This should result in a geometry with a single region, which will work for an incompressible analysis.
Cheers