I purchased an Assistant, but it hasn't been added to my account

If you are an iOS user, then you should be able to easily fix the problem yourself.
First, try purchasing the Assistant again - a pop-up screen should appear and allow you to redeem it for free.
If it does not appear, then please contact Apple for a refund and purchase Assistant again normally.

Tip: If you are Android user, contact Customer Support and attach purchase receipt. (Support in settings tab/e-mail: [e-mail])