Well you are asked to return a string in the first place

as far as I understand, single words in a line do not need space at the end, so this must be the reason. I've read about similar issues here. See instructions: 'Strings with one word do not need gaps'

i know it's not a lot, but please use the spoiler tag when discussing code, thanks. i have added the tag.

I assume you mean when selection 'Rank up', right?
It depends a bit on how many katas are actually available in the chosen language.

You don't say which language, I assume Python. Tests use lists.