For new WordPress users, my short Goggle search did not find anything on this but this is what’s going on.

If you click on “Comments RSS” and you get a page saying “…could not update this feed because the source file is no longer available.” don’t worry.

I just means you don’t have any current comments attached to any of your post. Once someone comments on a post (and you approve the comment) you’ll see your Comments RSS feed.

Of course if you don’t delete the standard “Hello World” post and it’s comment you’ll never come across this problem.

Hope this helps.