How do you test StoreKit subscription purchases on Apple Watch?

Using a modified version of the following example

    var body: some View {
        NavigationSplitView {
            BackyardList(isSubscribed: isSubscribed, backyardLimit: passStatus.backyardLimit, onOfferSelection: showSubscriptionStore)
            .navigationTitle("Backyard Birds")
            .navigationDestination(for: Backyard.ID.self) { backyardID in
                if let backyard = backyards.first(where: { $0.id == backyardID }) {
                    BackyardTabView(backyard: backyard)
                }
            }
        } detail: {
            ContentUnavailableView("Select a Backyard", systemImage: "bird", description: Text("Pick something from the list."))
        }
        .sheet(isPresented: $showingSubscriptionStore) {
            SubscriptionStoreView(groupID: groupID)
        }
        .onInAppPurchaseCompletion { _, purchaseResult in
            guard case .success(let verificationResult) = purchaseResult,
                  case .success(_) = verificationResult else {
                return
            }
            showingSubscriptionStore = false
        }
    }

(from Apple's sample code demonstrating in-app purchases), I'm unable to complete a sandbox purchase on Apple Watch. I get the error in the UI

Unable to Purchase App

Sign in with your Apple ID from the Apple Watch app on your iPhone

and printing purchaseResult outputs failure(StoreKit.StoreKitError.unknown). An Apple ID is signed into Settings on iOS, as well as the Apple Watch app. This occurs whether or not a separate sandbox Apple ID is signed into Settings under App Store. The subscription options UI appears as expected before attempting to purchase one.