Build a Product Landing Page: Navigation cannot be fixed

HTML-CSS
1

I used a flexbox on my navigation and now it cannot be in a fixed position, please help!

https://codepen.io/MUNACHI/pen/pZPdMe?editors=0100

Chrome

User Agent is: Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/responsive-web-design/responsive-web-design-projects/build-a-product-landing-page

2

If I give the header a fixed position and a width of 100%,
then the video area only needs some margin or padding.

A fixed position puts the element out of the normal flow
and puts the element “above” it, from a three dimensional view (depth, z-axis).