Technical documentation page- help

Hi, i don’t understand why the flex box it is not working in this code.

I wanna have an aside bar on the left with a width of 25% of the page, and the other section should have the rest of the space.

Proyect:
https://codepen.io/silvamaxi93/pen/abdNgpg?editors=1100

When there is no content the element’s height is zero, as we may expect.
Write on them or set the height using css.

Plus, you forgot colons in one flex item definition.