I get an error message when trying to purchase an in-app purchase. I am connected to the internet, however.

Please make sure you are a logged in with the Apple ID that you have used to purchase the app. In-App-Purchases can only be bought with the same Apple ID that was used for buying the app.

1) Go to the App Store app > tap user avatar in the top right corner, scroll to the bottom of the sheet and sign out your current Apple ID.

2) Log in with the Apple ID that you have used to purchase ProCamera.

3) Now you should be able to purchase HDR and LowLight Plus in ProCamera.

A connection to the internet is needed in order to purchase or restore an in-app purchase.