[Plugin: Google XML Sitemaps] “no input file specified” error

I installed the plugin according to the directions, and I’ve created my sitemap files, but it still tells me they cannot be found or aren’t writeable — which I find odd, because I have them located in two places just to be safe:
(i.e. chrisbrownweb.com/sitemap.xml & chrisbrownweb.com/blog/sitemap.xml), and the sitemap.xml.gz file is in both directories too.

I’ve done EVERYTHING I can think of.

Why am I getting an error that reads “no input file specified” every time I try to build the sitemap?