You didn't delete second copy line 'copy /Y \Work\BabaCAD...' as I wrote in the last reply. Also check that you compiled the project and you have BabaCADNetDemo.dll in output folder (open windows explorer and see if .dll exists in \Program Files\BabaCAD\BabaCAD...\NetProjects\BabaCADNetDemo.dll
Also be sure that you run Visual Studio as Administrator (find VS icon, right click and choose 'run as Administrator', it needs admin rights to copy .dll to \Program Files\BabaCAD... folder.
If you can't get it work (but it must work if everything done correctly), just delete everything in post-build events and manually copy .dll from output folder to \Program Files\BabaCAD\BabaCAD 2019...\NetProjects folder.