I am having difficulty seeing what you are trying to model here. There is are several opening that look like doors in a building. One of these the largest, that you marked 1 above, leads out of the domain. Is the volume to be simulated inside these "walls"? What would be the outlet?
You are in the Fluent solver. I mentioned you need to open it in Fluent Meshing. It looks like you are using Workbench and not the stand-alone. Since you built your geometry & mesh outside of ANSYS I see little reason to use workbench, but if you want to you would open it in the Fluent (with Meshing) component system not Fluent or Fluid Flow (Fluent). In the meshing the pull-down menus will be labeled File, Boundary, Mesh and some others. Here you can deconstruct your mesh as you need using the technique I described above.