Charles Ellicott Commentary Numbers 15:25

"And the priest shall make atonement for all the congregation of the children of Israel, and they shall be forgiven; for it was an error, and they have brought their oblation, an offering made by fire unto Jehovah, and their sin-offering before Jehovah, for their error:" — Numbers 15:25 (ASV)

For it is ignorance.— Rather, for it is a sin of ignorance, or an error. So also at the end of the verse.