Does the failure to perform the Hajj on behalf of a deceased mother, after the individual decided to leave their job in Mecca, constitute a sin?

There is no sin upon you for not performing Hajj on behalf of your mother, and you were not obligated to perform Hajj on her behalf merely by your intention, as an intention for Hajj does not necessitate anything. There is no blame upon you for not performing Hajj on behalf of your mother, unless you had vowed to do so. You should strive to honor her after her death by frequently seeking forgiveness and supplicating for her, and by giving charity on her behalf. If your mother did not perform the obligatory Hajj (Hajj al-Islam) and it became incumbent upon her during her lifetime, but she did not perform it before she died, then it is obligatory to use a portion of her estate for someone to perform Hajj on her behalf, provided she left sufficient assets.