Indexed, not submitted in sitemap

Hey,
I am facing a problem that from 08-08-2021 my sitemaps are not being updated and my posts are also not being indexed and I am losing the traffic because of that.
before I was using Yoast SEO plugin and I was not facing issues that much. but then I switched to Rankmath and I was hoping that it will be the best option for me.
but I am facing a serious issue regarding sitemaps, as I did every possible effort to solve this issue but it’s not helping me out.

I am facing several issues such as:

Crawled – currently not indexed
Excluded by ‘noindex’ tag
Discovered – currently not indexed
Duplicate without user-selected canonical
Alternate page with proper canonical tag

Even I tries robots.txt Tester too but this result was shown up:

You have a robots.txt file that we are currently unable to fetch. In such cases we stop crawling your site until we get hold of a robots.txt, or fall back to the last known good robots.txt file

Kindly help me out as I am losing my traffic
Thank You