Moshe said to Aharon: Come near to the Altar and perform the service of your sin-offering and your elevation-offering and provide atonement for yourself and for the people
Moshe told Aharon to perform the service of the sin-offering at the end of the inauguration period for the Tabernacle, and said to him concerning that offering, "provide atonement for yourself and for the people." But this is dufficult to understand: The sin-offering calf that Aharon brought only provided forgiveness for Aharon, and not the people! The people's forgiveness was provided by the goat offering, as the end of this verse states.
Ibn Ezra and Ohr HaChaim have asked this question, and suggested answers. It appears to me that the answer is this: Atonement is not possible if one continues to sin. Therefore, if the leader and teacher of the nation, whose behavior is naturally emulated, is a sinner, the nation will not be forgiven for their sins because of the likely prospect that they will continue sinning. Therefore, Moshe said that first Aharon must purify himself and ensure that he is clean of sin, and then others will emulate him and follow the path of Hashem Yisbarach and His holy Torah. This would prepare the nation to receive atonement. Then, the only sins to be concerned about would be those from the past, which can be forgiven through the goat offering.
We see here how anyone who is influential on other's behavior, such as community leaders or teachers, must be extremely careful to achieve and maintain the highest purity possible, for what they do determines whether or not the repentance of others will be acceptable.