It means you missed something, code and instructions are 100% accurate If you are not aware about liquid, I suggest please hire someone to implement this.
Yes, please confirm where you add this file code
( 2. Under section add this file using same name File )
Check this screen-shot https://prnt.sc/qaukou
If still if you have any issue, please drop email
Check the Shopify Banner Slider App, you can easily customize banner dimensions, apply it on full width. You can change slider animation, slider interval, and add unlimited images for display. You can select from 8 multiple exciting slider effects to make your store more attractive and eye-catching.