The description gives the example:

namelist([])
# returns ''

You must return an empty string if an empty list is given as the parameter. You know you can print the input parameter to the console, right? Your code fails because it attempts to pop from the empty list, leading to an error.

To control formatting of code in the comments, you need to use markdown, see here: https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet#code-and-syntax-highlighting