Chabad.org — English translation + Rashi
Torah Temimah points to Horayos 10a.
E-Daf.com — Horayos 10a
HebrewBooks / Shas PDF
English: If he sinned and afterward was appointed — he brings [an offering] as a private individual.
Pre-appointment sins remain ordinary — a sin committed before elevation to High Priest is atoned by an ordinary individual’s chatas.
Sins committed before elevation to High Priest are atoned by an ordinary individual’s chatas, not by the bull of the anointed. The special status of the High Priest and the corresponding bull offering begin only with the appointment itself. Any prior inadvertent sin retains the status of a private individual’s chatas and is not upgraded by later elevation.