Finally solved this. In my Sandbox users I can set the AppStore they use. Setting these to US allows the sandbox users to complete the in-app purchase. Frustrating if purchases only work under the US App Store that sandbox users can create under other stores.

Still happening with inAppPurchase in TestFlight on 15.5... When will this be fixed?

Question is old but for the benefit of others I had this problem with SwiftUI having no size for the UIViewRepresentable. I solved it by adding an intrinsicContentSize variable in the base view (not the UIViewRepresentable wrapper), that calculates the size based on the key subview frames and some padding. eg. swift override var intrinsicContentSize: CGSize { textContainer.frame.size + (mainViewPadding * 2) }