I bought a new Microsoft Lumia 640 XL and I need a little help?

So, I got this phone today and everything is ok, except for cortana… The cortana app doesn't exist in my "all apps". I know that pressing the Search button for 2-3 seconds enables Cortana but she doesn't answer very well… Let's say that I say "Tell me a joke", instead of Cortana saying a joke, it just says Searching for 'Tell me a joke'. What is up? I have windows 8.1 Update 2. Thanks in advance.

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