the mid surface extraction didn't seem to work. Use the offset surface command and make a surface (well 3 surfaces) and then delete the solid. Make a new config if you must. You would be well advised to split the surface so that the brackets have common edges with the big piece (so the nodes merge).
The reason for failure is the gussets' thickness is not as same as the main body (1.5mm). That's why the automatic conversion to shell for sheet metal does not work as you have two different thicknesses within one solid body. If you modify the gussets' dimension, it will work. Otherwise, you should create surfaces manually and assign proper thickness to them individually.
I think you should not use the sheet metal gusset feature in this part.
If you look closely to the created gusset, how are you going to make that? You never have enough material to create such a gusset from a sheet metal plate.
You should create two extra bodies, which will be welded at the sheet metal part to represent the gussets.
In this case the simulation study will work also.