Please double check that you named your verification file correctly (like Google told you) and that there are no spaces in front or after the file name. The GSiteCrawler can generate the verification file for you, you can just copy+paste the filename from the web page to the GSiteCrawler and let it upload the appropriate file. Otherwise you may also use the meta-tag verification system.

If it still does not work, make sure your server returns correct headers for existing URLs (200) and for missing URLs (404). You can check this by using a server header status code tool. Enter the URL to your verification file and check that your server returns "200 OK" as the status code. Then try a not existing page and verify that your server returns "404 Not found". If you recieved a "200 OK" again then you need to adjust your servers settings (or contact your hoster to do it for you). Please note that there are some hosters that block Google from accessing the site fully - in these cases you might also see this error message. If your hoster is blocking Google's access, then submitting a Google Sitemap file is futile, Google won't be able to crawl the URLs anyway. In this case I can only recommend moving to a hoster with a real understanding of the internet.