This one took me some time!
<atom10:link xmlns:atom10="http://www.w3.org/2005/Atom" rel="self" type="application/rss+xml" href="https://example.com/news.rss"/>The error message is not very informative when it tells you to add a <atom:link>, because it should be telling you to add an <atom10:link>.
Here is a demo of it at work: W3C Validator.