What's your CMS?

Your site is probably doing conflicting redirects, either in htaccess, 301, server side, etc.

You need to save it to the root directory, probably, without a www.

Your sitemap is probably redirecting to www ......sitemap....and google is expecting the root.

If your cms is doing this, you can manually change it.

But in the end.....you don't even need sitemap. I have never, ever, ever, ever, felt the urge to do one.

All of my urls are readily available to google.

My feeling is if google needs help, then your siteis beyond help.

Sitemaps are like those pedestrian buttons at the traffic lights.

Pushing the button does not magically instantly make the signal change....

But you see fools with a trigger finger...rapidly pressing the button.....getting nowhere.

Of course I have never felt the need to have a cms either....I chose to do my site exactly the way I want.

And you actually learn how to internet works.....

Paul

Are you sure that you are entering the url correctly?

Here I saw you are facing some issues with Sitemap.xml, my opinion about this issue is-

As you told you used Open cart Google sitemap map. May be there some issue, Generate manually by online tools, use www.xml-sitemaps.com this is the best Tools for XML sitemap.

May be you opencart version is old that why creating a issues, Upgrade it.

If after following above approach you are not getting success then you can consult with SEO expert or search engine optimization company that will definitely help you to rectify by this situation.