There might be 3 reasons this is happening:
1.) You’ve bought the full app directly – not first the free app with the in app purchase. If this is the case , you can download the full version here for free: https://itunes.apple.com/app/id435476174
2.) You’re not logged in with the same Apple ID – this way Apple doesn’t recognise the purchase. You can login with your Apple ID and go to the menu in the free app and scroll a bit down and click on ‘restore’ .
3.) You wanted to share the premium version through Family Sharing. Unfortunately, Apple’s official FAQ website informs they do not support in-app purchases within Family Sharing. If you want to get a free copy for your family members, please contact us via email@example.com and we will assist you with it. Please attach your purchase receipt in your email, so we can help you quicker.