There is a problem. They don't understand who He is and what He has been called to do. As such, they can't judge Him or His actions. He has come from God. The miracles and other signs have proven this. His authority is from God. Even though He appears to be bearing witness of Himself, in truth, He is only telling them what God wants Him to say to them. They are trying to impose earthly standards to God and the servant of God.

Jesus came from heaven and the throne of God. He knew and understood things that they could not comprehend. The Pharisees were of the world. As such, they had no ability to judge his testimony.

I know by what authority I act; I know by whom I am sent, and what commands were given me; but you cannot determine this, for you do not know these unless I bear witness of them to you.

We are to remember that Jesus came not of himself John 6:38; that he came not to do his own will, but the will of his Father. He came as a witness of those things which he had seen and known John 3:11, and no man could judge of those things for no man had seen them. As he came from heaven; as he knew his Father's will; as he had seen the eternal world, and known the counsels of his Father, so his testimony was worthy of confidence.

As they had not seen and known these things, they were not qualified to judge. An ambassador from a foreign court knows the will and purposes of the sovereign who sent him, and is competent to bear witness of it. The court to which he is sent has no way of judging but by his testimony, and he is therefore competent to testify in the case. All that can be demanded is that he give his credentials that he is appointed, and this Jesus had done both by the nature of his doctrine and his miracles.