I mailed support and got this:
In order to prevent spam and other types of abuse on Tumblr, that feature is locked on new accounts. After you've used the account for a while to do things like Following other blogs, Liking posts, and customizing your blog's appearance, the feature will be unlocked so you can use it. We apologize for the inconvenience.
However I have been using the service for over two weeks, customised my blog, been following, blogging, reblogging, liking and commenting and still the same problem.
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