Post part of site map “couldn’t fetch”

I submitted my sitemap to Google Console using the format given by the Yoast plugin – sitemap_index.xml

The status is “Sitemap index processed successfully” and “success” however when I click on this sitemap it opens up 6 variations of the sitemap. I understand this is normal but the problem is that the /post-sitemap.xml (presumably the part with all of my blog posts) says “Couldn’t fetch” and when I click on that I get a further error message: “Sitemap could not be read”.

This has been like this for several weeks now and I’ve tried clearing caches, re-submitting but nothing changes.

Can anyone help me with why this is happening and what I need to do to fix it?

Thank you! ??

The page I need help with: [log in to see the link]