How did the Messenger, may Allah bless him and grant him peace, command the Sunnah, even though a command implies obligation, and the definition of the Sunnah does not necessitate punishment for one who abandons it?

The meaning of the word "Sunnah" varies depending on the technical usage. For jurists, it means what is recommended (mustahabb) and desirable (mandub), actions for which one is rewarded if performed, but not punished if abandoned. For Hadith scholars, it refers to everything attributed to the Prophet (peace and blessings be upon him) in terms of sayings, actions, approvals, or descriptions. This can include what is obligatory, recommended, permissible, disliked, and forbidden. For jurists and theologians discussing creed, it can mean the opposite of innovation (bid'ah), i.e., the straight path that the Prophet (peace and blessings be upon him) and his Rightly Guided Caliphs followed in beliefs, actions, and sayings. As for "whoever turns away from my Sunnah," what is meant is the way or path, and turning away from it means shunning it.