Description
When making an in-app purchase for an app installed using Volume Purchasing, it is expected for users to see "This Apple Account cannot be used to make purchases."
Apple does not allow in-app purchases through volume purchased apps because app licenses obtained through Apple Business Manager can be both deployed and reclaimed by organizations, making it difficult to track the ownership of in-app purchases.
In-App Purchase Options
In order to get the in-app purchases on the device we can try one of the options below:
Download the app from the App Store using personal Apple Account and make the desired in-app purchase. The in-app purchase will then be associated with the Apple Account used to make the purchase.
Note: Managed Apple Accounts cannot download apps from the App Store.
Reach out to the app developer and see if they offer a version of the app that includes the in-app purchase functionality. If so, you can then purchase that app version in Apple Business Manager and deploy using volume purchasing license assignment.
Reach out to the app developer to see if the app offers any other way to pay for the desired functionality without using in-app purchases.