Use bare return, don't add that print() statement
Edit: I flagged your comment as spoiler as it might gives a hint to the other warriors

Have you tried adding the relevant import statement to the top of your solution - using System.Collections.Generic;?

Which language ? What do you mean with the compiler would not detect List variable? This kata has been solved about a 150_000 times, it's reasonnable to suppose your code is the culprit a priori... Or please give clear examples of fails.