Hi Ofer,
Probably, Stephen is right that you have inputted an incorrect command line parameter.
I suspect that you get this message as the path to your msi file is specified in an incorrect way. Please pay attention to the fact that all the files specified to be opened in the command line must be referenced using the full path to the file. If the file path contains spaces, it should be enclosed in quotes "". Look at the example:
//...
var MSIpath = "C:\\Users\\Ofer Vil\\App.msi";
//...
TestedApps.Add("msiexec.exe /i" + ' "' + MSIpath + '" ');
If this does not help, please post here the problematic code.