Highlight and contouring for darker skintones tends to be a bit tricky compared to lighter skintones so it's important to perfect the steps.

Casting light and shadow perfectly (essentially what highlighting and contouring does) on the face requires practice to nail it perfectly over time.

Contouring is supposed to cast 'shadow' of parts of your face (and vice versa with highlighting). Learn how to get a flawless highlight and contour for dark skin tones in easy steps from the tutorial by Kluermoi above.