Aside from a missing closing curly brace, and the Flight Simulator-specific reference, that compiles on my machine.
As denoted by"Message" in the "Severity" column, those are merely suggestions, not issues that would keep your plugin from working.
Do note that learning how to program in C#, especially using external libraries and interfacing directly with other applications, would probably be considered outside the scope of VoiceAttack itself.
There are resources available online, and courses you can take, both on C# and programming in general, if you want to dive deeper into that.