Make curves solid

I have this structure, however, I can only seem to extrude two walls instead of making the structure solid. Do I need to loft the faces or cap, or something else, not sure where I’m going wrong.

Thanks (38.0 KB)

Extrude curve will not create a solid from a curve.

You will need to cap and then cut out the inner geometry with a boolean difference component.

The alternative is you create a boundary surface with the cutout before you extrude and that will create a solid from the extruded surface.