Generating a block Geometry

Hi, I'm fairly new to this domain so I'm not sure if my methodology is correct. I've used Gambit before but I decided to use and Open Source program instead.
I basically want to generate a mesh (of some height) over a digital elevation model (DEM). The DEM is made up of a matrix of points (in essence) and so to test my ability to simulate fluid (in my case air) movement I plotted my own points.
Once I had a grid of points (on the floor of the DEM and points above to form a boundary layer) I used the New Entity > Blocks > Quadrangle Face option to join four adjacent points together to form tiles until eventually I had a closed object.
I used the Boolean > Fuse option to combine the quadrangle shapes together (which took allot of effort since you can only fuse two objects together at a time?) and finally the New Entity > Group > Create option to define walls.
Am I going about this the wrong way? When I try to compute the mesh I end up with an error
Any suggestions/hints would be deeply appreciated.
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