Feature Wall on the Home Store

When you choose walls, and pick a wallpaper 4 green panels show up. Choose the green panel on the side you want whichever panel. :)

I can’t find it anywhere on the home Store 

It is not in the home store as "featured wall paper", you can pick any wallpaper you want or can afford, then when you see 4 green panels of the chosen room de-light 3 of them so you only change one side of the room to make a featured wall

It’s not an own category.  In the tab “Walls” choose  a normal wallpaper and tap into the room you want to decorate. Then you will see the green fields and by tapping one, two or three of the green fields you create a feature wall.