A timer from System.Timers will tick on a non-UI thread, so you must use a Microsoft.UI.Dispatching.DispatcherQueue somehow.

You can use the one on the current Window or get one from the current thread (if it has a dispatcher queue) using the DispatcherQueue.GetForCurrentThread method.

Then your code can be written like this:

private void Timer_Tick(object sender, EventArgs e) => DispatcherQueue.TryEnqueue(() =>
{
    timerLabel.Text = DateTime.Now.Subtract(startTime).ToString(@"hh\:mm\:ss");
});

Note that you can also use a timer provided by the DispatcherQueue itself with the DispatcherQueue.CreateTimer Method which will automatically tick in the proper thread.