When submitting a sitemap in Google search Console, it seems you need to stick with all HTTP or HTTPS URLs and not mix them. I haven’t tested this myself, but I know our sitemap here only references HTTPS URLs on a verified HTTPS profile and it doesn’t have this issue.
But when webmasters try to mix and match, Google Search Console serves up an incorrect namespace error.
John Mueller of Google responded to such a complaint on Twitter saying “woops, I’m sure we can fix that :).” But this has been an ongoing issue for a long long time. Here is a thread from months ago on Google Webmaster Help.
Here is a screen shot of the error: