Several things might have happened.
1. You may have downloaded ALICE on a device with a different Apple ID. In that case, you would have to pay again because Apple ties app purchases to the Apple ID that is logged into the App Store at that time.
2. It is possible that you are trying to make an in-app purchase for additional content inside of ALICE. When you purchased ALICE the first time, the General and Medical categories were included. If you want to use a different category, you need to purchase it.