How long have you had him? Where is he doing the growling and nipping, in his cage? What are you doing when he does growl and nip at you?

It’s very easy to “taint” a clicker command and render it useless with just one reprimand. You could try clicker training him with a new command for a similar behavior and reward. It’s harder to train him not to do something (growling and nipping) than to do something, so it will take longer.

He may be afraid of your hands when you approach him. Try to keep your fingers together when you reach for him (spread fingers resemble a predator’s claws) and reach with your palm up, and low, instead of coming down onto his head from above. Let him come to you as you hold a treat and be still. Also, have you sat on the floor with him and let him explore you? A bunny is curious, but shy of movements toward them. Try lying on the floor in front of the TV or reading a book. Ignore him. Put a treat in your hand or pocket. Let him sniff you all over and continue to ignore him. When he gets to your hand, open it gently and let him find and snatch the treat. You are trying to build trust and make him associate you with safety in your company and with good surprises, not bad ones (as in being picked up.).