As I sems the issue is the difference in the methord Signature

.Callback(() => FakeCaptureTargetScreen(
        "Test", // Replace with Real value
        100,   // Replace with Real value
        50,   // Replace with Real value
        bmp => {
            // Process the captured Bitmap (bmp) here
            Console.WriteLine("Bitmap captured!");
        }
    ));

As for the second

Can it be simpified?

I did some reserch ans found specify argument matchers like It.IsAny() are requird for each parameter and Moq does not provide a built-in shorthand to match any value for all parameters. Each parameter must be specified explicitly. For example, your setup must be written as

_mockScreener
    .Setup(x => x.CaptureTargetScreen(
        It.IsAny<string>(),
        It.IsAny<int>(),
        It.IsAny<int>(),
        It.IsAny<Action<Bitmap>>()
    ));