You part has issue with the flat pattern. Fix that everything would be OK.
I have had a look at the model and I think it is because of the edits you have made to the sheet metal.
In the model, if you try and flatten the part it does not work, because you have added and merged non-sheet metal features to the part.
If you roll back to a point where the model will successfully flatten;
The model will successfully mesh;
Obviously this mesh is incorrect because I have suppressed features from the model, but essentially, ! if the part wont flatten then the automatically generated shell mesh will not be correct. SOLIDWORKS will automatically try to form a mid-surface for any sheet metal bodies in a simulation, and because of merge entities it fails to generate successfully. If you edit the definition of the shell you can see what SOLIDWORKS has created. The big surface is caused by boss extrude 18 I believe
To get around this I would manually create the mid-surface of the model, or modify the geometry to create a flattenable model.
I have attached a partly completed model created using the mid-surface command, but you can see that the mesh is starting to look like it should;
Hope this helps!
Modifed.zip 769.9 KB