Sandbox subscription purchase fails with ASDServerErrorDomain 3504

Type: Auto-renewable subscription Sandbox Storefront: USA

StoreKit successfully loads the subscription product and returns its correct name and price (US$29.99). However, every purchase attempt fails:

ASDServerErrorDomain Code=3504 AMSServerErrorCode=3504 "Item not found" client-environment-type=Sandbox storefront-country-code=USA

This reproduces both in a TestFlight build and in iPhone Developer Settings > Test In-App Purchases.

Please verify the app-product linkage and Sandbox commerce availability for this subscription product.

Purchase did not return a transaction: Error Domain=ASDServerErrorDomain Code=3504 "找不到此项目。" UserInfo={storefront-country-code=USA, AMSServerErrorCode=3504, client-environment-type=Sandbox, NSLocalizedFailureReason=找不到此项目。}
HKIPO-IAP 11:12:33.712 purchase error: type=StoreKit.Product.PurchaseError, domain=StoreKit.Product.PurchaseError, code=1, description=無法使用項目, failureReason=目前無法使用此項目。, underlying=nil, userInfo={}