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CSS position problem

https://www.codecademy.com/courses/learn-css/lessons/css-display-positioning/exercises/css-position-fixed?action=resume_content_item


I’ve already seen in youtube. He able to use the property sticky (as function fixed & absolute) . But, as i do in codeacademy compiler/ in my compiler, i cant do the same thing as he do. what’s the problem?