Sitemap index errors

[Koala_Steamed]

I'm getting a whole lot of errors 2000 "invalid url" and 2000 "Missing thumbnail" in my google search console. Not sure if this is an issue with the xenforo generated sitemap or if I've typed something in wrong. At the moment we also don't have a robots.txt not sure if that should be added?

Any help with this would be much appreciated