Welcome to the community. 
Make sure you, at the very minimum, have .NET Framework version 4.7.2 installed on your machine. Preferably you should have version 4.8 though.
If you have, reinstall it anyway in case it’s corrupted.
The official download link for it is in this thread, along with some other troubleshooting steps: I can’t Install WeMod / "There was a error installing the application" - #2 by Ravenfyre.
Do you have multiple desktops, and if so have you checked WeMod wasn’t opening on another desktop?
Which antivirus do you use?