SEO – Google could not retrieve the sitemap

Sitemaps are a necessary part of our web work. And we need them to love Google more. But for about a month, I can not submit sitemaps to the Google search console.

I've used https://technicalseo.com/tools/fetch-render/ to validate and successfully pass. I used the sitemap validator and this one did not show any errors. It seems that something from the search console has to be wrong.

Yesterday I started another site and used the same WordPress plugin to generate the sitemap. Then I used the same Gmail ID to sign in to the search console and tried to add a property, and this time it worked.

If I submit a new Sitemap that could not be retrieved, and I try to resubmit an existing Sitemap, it succeeds, but the last reading date is 6 months ago, so I assume that it will not work.

What are possible solutions. The live test says it can not retrieve because it is not an index. I think there is nothing wrong with keeping sitemaps without an index, as sitemaps should not be displayed in the search. Infact my second domain also got sitemaps with noindex

I use WordPress PHP 7.3 and HTTP / 2

My sitemap URL is https://milyin.com/sitemap.xml

My HTTP headers are:

HTTP/2 200 
server: nginx
date: Thu, 22 Aug 2019 12:23:21 GMT
content-type: text/xml; charset=UTF-8
vary: Accept-Encoding
x-robots-tag: noindex, follow
link: ; rel="https://api.w.org/"
last-modified: Wed, 21 Aug 2019 14:03:48 GMT
etag: W/"59184deaadc3de11f553f5a8fbaac7f0"
x-httpd: 1
x-robots-tag: index, follow
host-header: 192fc2e7e50945beb8231a492d6a8024
x-proxy-cache: MISS
content-encoding: gzip