This error can occur in the following cases


Case 1: NetSim application not having sufficient previliges


Also make sure that you run NetSim in Admin mode, by right-clicking on the NetSim shortcut icon in the desktop and selecting Run as Administrator


Case 2: When interfacing with MATLAB


Make sure that the following directory is in the environment variable PATH: 

<Path where MATLAB is installed>\bin\win32


For eg: "C:\Program Files (x86)\MATLAB\R2019b\bin\win32"


By default, only the path till the bin folder of MATLAB will get added to the path variable. i.e 

"C:\Program Files (x86)\MATLAB\R2019b\bin"