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.
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.