Error message : Call to undefined function query_posts()

Hello,

I wanted to create a template for a custom RSS feed.
I found this tutorial: http://www.wpbeginner.com/wp-tutorials/ … wordpress /

I have a problem with the template because when I try to open the new feed file, I get this error message:

Fatal error: Call to undefined function query_posts() in C:\xampp\htdocs\mywebsite\wp-content\themes\mytheme\rss-feedname.php on line 6

I tried with another template: http://yoast.com/custom-rss-feeds-wordpress/
But same error message (except that it is line 20 on this one).

How to solve this problem “all to undefined function query_posts()”

Thank you