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If your ask page returns this issue, you haven't enabled the 'Let people ask' feature on Tumblr. You must enable “Let people ask” feature on your Dashboard > Setting to use. If you removed your blog before, and created again, it may take time to put it back.
What's the difference between 'Messaging' and 'Ask'?
The 'Ask' feature simply allows you to send a text message to your Tumblr inbox while the 'Messaging' feature works just like 'Message' from Facebook – it's a Tumblr version of a chat platform. There's another form of interacting with your followers: Submission. Submission on Tumblr allows your followers, or visitors to post images, audios, texts, or any sort of post types Tumblr supports onto your blog.