A baby bird sometimes only squeals. The effort to communicate with you has been rewarded. To teach the lessons listed above and others designed to enhance the bird's strengths enriches my life and the parrots. Your expectations should be reasonable when teaching a parrot. No one can teach a parrot to never scream.
Parrots do that.
It is instinctual behavior. Modifying the behavior like creating conditioned screaming is the reasonable way. One disappointment to me is my male Moluccan Cockatoo has never taken food from my hand. It is not reasonable for me to insist he do this, but I am consistent and offer him food at least once each day from my hand. Be careful of teaching a lesson accidentally.