How to add a timer to C# Service

I can't build a test right now, but my first thought is that the issue is that your Main is completing (there's nothing holding the main thread). *

Instead of an event-based timer, why not use Task-based asynchronous code.

Not tested, but something like:

async static Task Main()
{
    while (isRunning) // A token or flag to hold the application running until the service is stopped
    {
        // do work

        await Task.Delay(sleepTimeInMs);
    }
}

Also, your OnStart and OnStop methods are extremely similar; I would look at taking the shared code to a function (e.g., runPowershellScript).

* RobIII is definitely more correct on this point. The issue is that your Timer is no longer referenced at the end of Main, so is being collected.

I assume the service continues to say it's running, as it's life-cycle managed by SCM.

Regardless, my suggestion is one way to solve it.