John Gill Commentary Numbers 15:27

"And if one person sin unwittingly, then he shall offer a she-goat a year old for a sin-offering." — Numbers 15:27 (ASV)

And if any soul sin through ignorance
Any private or particular person, by breaking any of the above commands, or any other, not rightly understanding them, or not adverting: to the circumstances required in the manner of performing them:

then he shall bring a she goat of the first year for a sin offering ;
which differed in this from the sin offering of a congregation that sinned through ignorance; that was a kid of the goats, whether male or female, but this was to be a female goat and of a year old.