In John 8:15 Jesus says he is “not judging anyone,” but in 8:26 he says “I have many things to speak and to judge concerning you.” Is there a contradiction between these two phrases, spoken to the same group of people?
πολλὰ ⸀ἔχω περὶ ὑμῶν λαλεῖν καὶ κρίνειν,* ἀλλʼ ὁ πέμψας με ἀληθής ἐστιν (in John 8:26, NA28)
Jesus had many things he could have judged them for, but he held the judgment back because that wasn't the reason the Father sent him.
Note: following 8:15, Jesus said,
Yet even if I do judge, my judgment is true, for it is not I alone who judge, but I and the Father who sent me. (John 8:16, ESV)
Also look at the context of 8:26. Jesus followed the Father's example. For example, God revealed himself to Pharaoh with the ten plagues knowing that Pharaoh would not have a lasting repentance. But, Pharaoh is under judgment because he was given the opportunity and rejected it. Thus, Jesus told them how he would answer their question, and by their response they are judged.
25 So they said to him, “Who are you?” Jesus said to them, “Just what I have been telling you from the beginning. 26 I have much to say about you and much to judge, but he who sent me is true, and I declare to the world what I have heard from him.” 27 They did not understand that he had been speaking to them about the Father. 28 So Jesus said to them, “When you have lifted up the Son of Man, then you will know that I am he, and that I do nothing on my own authority, but speak just as the Father taught me. 29 And he who sent me is with me. He has not left me alone, for I always do the things that are pleasing to him.” 30 As he was saying these things, many believed in him. (John 8:25–30, ESV)
Appendix: more related scriptures
But Pharaoh said, “Who is the LORD [מִ֤י יְהוָה֙], that I should obey his voice and let Israel go? I do not know the LORD, and moreover, I will not let Israel go.” (Exodus 5:2, ESV)
The Egyptians shall know that I am the LORD, when I stretch out my hand against Egypt and bring out the people of Israel from among them.” (Exodus 7:5, ESV)
Then Pharaoh called Moses and Aaron and said, “Plead with the LORD to take away the frogs from me and from my people, and I will let the people go to sacrifice to the LORD.” 9 Moses said to Pharaoh, “Be pleased to command me when I am to plead for you and for your servants and for your people, that the frogs be cut off from you and your houses and be left only in the Nile.” 10 And he said, “Tomorrow.” Moses said, “Be it as you say, so that you may know that there is no one like the LORD our God. 11 The frogs shall go away from you and your houses and your servants and your people. They shall be left only in the Nile.” (Exodus 8:8–11, ESV)
Truly, I say to you, it will be more bearable on the day of judgment for the land of Sodom and Gomorrah than for that town. (Matt. 10:15, ESV)
And you, Capernaum, will you be exalted to heaven? You will be brought down to Hades. For if the mighty works done in you had been done in Sodom, it would have remained until this day. 24 But I tell you that it will be more tolerable on the day of judgment for the land of Sodom than for you.” (Matt. 11:23–24)