BTW... This next bit of information could be helpful.

I have Fabrication 2016 and 2017 installed on my computer.

Fabrication CADmep 2016 runs on top of AutoCAD.

Fabrication CADmep 2017 runs on top of AutoCAD MEP.

The Visual Studio project has only been defined to run using the AutoCAD 2017-related DLLs using .NET framework 4.5.2

The debug startup is:

Start external program: C:\AutoCAD 2017\acad.exe 

Command line arguments: /ld "C:\Program Files\Autodesk\Fabrication 2017\CADmep\CADmep21_x64.arx"