Receive error when adding feed URL in Google Merchant Center

I receive an error that the file URL is not valid when I add it into Google Merchant Center. I have created an XML file in your plugin. Then I go into merchant center and follow the instructions. I’ve tried different variations such as adding the URL with and without the XML file name at the end and with and without the file name added in the space above in merchant center. None work. Do you have any advice for how to resolve this? Thank you! Mark