Who made the request to sit beside Jesus in His kingdom?  It was the mother who asked on different occasions. Matthew 20:20-21 and Mark 10:35-37

1. Matthew 20:20-21, “Then the mother of the sons of Zebedee came to Jesus with her sons, bowing down and making a request of Him. 21 And He said to her, “What do you wish?” She said to Him, “Command that in Your kingdom these two sons of mine may sit one on Your right and one on Your left.”
2. Mark 10:35-37, “James and John, the two sons of Zebedee, came up to Jesus, saying, “Teacher, we want You to do for us whatever we ask of You.” And He said to them, “What do you want Me to do for you?” 37 They said to Him, “Grant that we may sit, one on Your right and one on Your left, in Your glory.”

Both are correct.  It is reasonable to assume that the mother first approached Jesus and asked Him about her sons.  Later, they approached Jesus with the same question.  Neither statement excludes the possibility of the other.  Therefore, it is not a contradiction.