Hmm - read the error message again...
Your method signature requires to return an IComparer for iperson the list.Sort() method returns nothing (void)..
What you do here is creating and sorting a List you don't obtain or create a Comparer!
Though, You can give the Sort method a comparer you want to use.
As far as I understand your requirement you should return a special Comparer implementation not create a list nor sort it.
You find an example how to implement and use different Comparers there.