Help Needed! user story confusion

So I have finished my product landing page however whenever i try to keep the navbar at the top of the viewport using “position: fixed;” it doesn’t come out right.
Also for “user story #15” I don’t which media query to use i’m still quite confused with the concept .
Any help will be much appreciated

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.132 Safari/537.36.

Challenge: Build a Product Landing Page

Link to the challenge: