Hey @tuan123 , Whenever you need help with any of the Freecodecamp challenges, use the “Ask For Help” button on your challenge page. It gives the link to the challenge as well as posts the code properly here on forum.
<a href="#footer" >Jump to bottom</a>
as the challenge states a tag must be targetted to footer
as you can see above href attribute is targeted to id"footer" and the target="_blank "is removed
finally add id attribute to footer tag.
hopefully u found the solution