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: firstname.lastname@example.org)