Albert Barnes Commentary Leviticus 5:17

"And if any one sin, and do any of the things which Jehovah hath commanded not to be done; though he knew it not, yet is he guilty, and shall bear his iniquity." — Leviticus 5:17 (ASV)

Though he wist it not - Ignorance of the Law, or even of the consequences of the act at the time that it was committed, was not to excuse him from the obligation to offer the sacrifice.