If you check your Google Search Console frequently for indexing status and health.
you might come across a situation where your sitemap submitted will show couldn't fetch sitemap error
As per many forum discussion, its kind of bug if your sitemap is opening in XML format in the browser everything is fine
Checklist to make sure everything is fine:
Manually Open Sitemap if it's working fine or not
Check coverage report to see the URLs submitted are indexed or not
If all the URLs inside sitemap are indexed Stop worrying about it**
Those errors will go away automatically
We have seen this many times in multiple people's account.