404s are common, even if the pages do exist, sometimes other temporary issues can cause 404s, server load or maintenance issues going on for example while Google was crawling.
Specifically however, these URLs are RSS feeds, have you editing RSS options or added any plugins that might alter RSS feeds in any way?
You can see the pages work on the demo:
https://startupwp.com/demo/feed/
https://startupwp.com/demo/comments/feed/