It means that the sitemap is not reachable by google. Check if your sitemap exists and that you gave the correct address. Also check the permissions for it.
This is the exact message that I get: Sitemap does exist. Why is google not able to detect it? The address given is also correct for the sitemap. same as the example given below "We've detected that you submitted your Sitemap using a URL path that includes the www prefix (for instance, http://www.example.com/sitemap.xml). However, the URLs listed inside your Sitemap don't use the www prefix (for instance, http://example.com/myfile.htm)."
There can be numerous reasons why the red cross is displayed, not just that the xml sitemap cannot be found. If there are any errors in the syntax of the file or certain errors with the URLs listed it could cause the red x to show up.
You are mixing with-www and non-www URLs. Settle on using one and use it allover website, sitemap etc.