Your instructor was probably unaware.
Mathematica is not Excel or Word. Despite the fact that you can write programs, it is not the front-end text editor for a coding utility. Mathematica is designed for performing mathematical operations. You enter input, you execute, and you get output. You want a history of that so you can check it later. After you execute, there's nothing to undo. You are finished with that input command, and the output is whatever it is. If you made a mistake, you go fix it and run it again.
Undo doesn't edit command logs. It edits active text.
_________________ Buckle your pants or they might fall down.
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