Any Catholic Christian who has received the sacrament of Baptism and is in a “state of grace” can be admitted to Confirmation [1306-1311, 1319]
To be “in the state of grace” means not to have committed any serious sin (mortal sin). By a serious sin a person separates himself from God and can be reconciled with God only by making a good confession. A (young) Christian who is preparing for Confirmation finds himself in one of the most important phases of his life. He will do everything possible to grasp the faith with his heart and his understanding; he will pray alone and with others for the Holy Spirit; he will reconcile himself in every way with himself, with the people around him, and with God. Confession is part of this, since it brings one closer to God even if one has not committed a mortal sin.
From YOUCAT, page 122-123