Rss Feed problem

When I try to burn my feed, it gives an error “The URL does not appear to reference a valid XML file. We encountered the following problem: Error on line 2: The processing instruction target matching “[xX][mM][lL]” is not allowed.”.

Please Help. My website is https://ergontronblog.com and the feed I am trying to burn is https://ergontronblog.com/?feed=rss2