sometimes google returns "couldn't fetch" wrongly, just check whether the pages are indexed and if they are, you don't need to worry about that message.
If they're still not indexed... well, there's probably something wrong with your sitemap... or maybe your pages are set to noindex by a SEO plugin or something... or maybe you've selected "Discourage search engines from indexing this site" in your WP > Settings > Reading menu... there could be a lot of issues, you might even have submitted the wrong URL of your sitemap, or made a typo when typing the URL in, or whatever.... lots of reasons...