Your feed couldn't be validated because there was a problem downloading it from the web server. You should try to diagnose this using a web browser, and make sure that the URL you supplied resolves to an accessible file.
This usually means that the URL was wrong, or that permissions on the server don't allow us to fetch that file. The error shows the message that the server sent, which may help.
Make sure this URL can be downloaded with a browser, then try again.
Not clear? Disagree?
You might be able to find help in one of these fine resources.