If you see a pop-up alert that says, ”This app is no longer shared with you. You need to purchase it from the App Store," it is likely happening due to the app being purchased under a Family Sharing plan. Apple's Family Sharing requires that you maintain the same credit card on both Apple ID accounts. If this changes, you may need to repurchase OnSong. This is due to how the App Store functions and is not something that OnSong can affect.
Please try the following: on your family member's device, go into the App Store > Profile (person silhouette icon in the upper right-hand corner) > Purchases > Family Purchases > Tap on member > Find OnSong and download.
To learn more: https://appleinsider.com/articles/18/03/06/watch-share-purchased-iphone-or-ipad-apps-with-family-members-with-icloud
If you continue to have trouble, you will need to contact Apple since all purchases and provisioning of apps are handled by Apple and the App Store.