Q: Family sharing asks to pay for app

You are using the same account (or the account is included in family sharing and you are looking at that account's purchases), and you are using the Purchased tab in the App Store app on the second device to redownload it : Share purchased content with Family Sharing - Apple Support ?

If it shows in the Purchased tab for redownloading then what happens when you tap on it to download it ? If you are getting a message then what is the full text of it ?

If you are referring to in-app purchases then they are not included in family sharing : What types of content can I share with Family Sharing? - Apple Support

What can't be shared :

• Songs added to iTunes Match from outside the iTunes Store.
• Individual Apple Music memberships and other subscriptions, such as magazines and newspapers.
• In-App purchases.
• Items that are no longer available on the iTunes Store.
• Items that you or another member of your family group has hidden. Learn more about hidden purchases.
• Some apps from the App Store. Find out if your app is shareable.
• Ringtones.

The app maybe, but in-app purchases done within apps are not.