Probably because you’re missing a space to separate the arguments:
enable_admin.StartInfo.Arguments = "user " + "administrator " + "/active:yes";
^----
enable_admin.StartInfo.FileName = @"C:\Windows\System32\net";
disable_admin.StartInfo.Arguments = "user " + "administrator " + "/active:no";
^----
disable_admin.StartInfo.FileName = @"C:\Windows\System32\net";
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solved How do i use net user inside a C# console program? [closed]