I've already encountered that problem. You just simply have to put slash to your permalink of your sitemap. For example: sitemap.xml/ as you can see it has slash at the end of the permalink.

So I've been creating a lot of Tumblr accounts with very high PA(40-80). I've already tons of them around 200+ accounts. All of them were banned. They said it's because of spam or primarily affiliate marketing contents. But some of the accounts don't even have any posts yet, but they still ban my account which why I consider Tumblr as bullshit. Is there any reasons for me to get banned? Is it because of multiple accounts in one IP address? Or was it something else?