With gloves on, soak your hands in soap water and then practice knot tying. In the actual surgical field, if you've come in contact with any blood your hands will be slippery and knot-tying will be very difficult from your usual dry, bare hands knot-tying feel.
You can definitely practice your knot tying outside the OR but in the end, I think closing on a real patient is the best practice you will get because the tissue texture and makeup of human dermis isn't comparable to pig/chicken skin.
Not to mention, patients' skin architecture is different too. For example, closing a 89 year old with multiple past surgeries is WAY, WAY different (in my opinion, harder) than closing a 30 year old with no surgical history because the 30 year old's skin is tighter and more stronger than the 89 year old's.