Sitemap could not be read – Search Console

I have a custom post type on my website and some few custom taxonomies. On the SiteMap settings page, I have enabled generation of sitemaps on every taxonomy. Unfortunately, one of the most important sitemaps for my website could not be read on search console. So I tried yandex xml validator and it also could not fetch the sitemap.

I used developer tools to view, copy & paste the sitemap on yandex xml validator and it read that it was okay, the only problem is that the url appears in-accssible by both yandex & google.