The constant SKErrorPaymentCancelled has been removed from the SDK as of iOS 9.3. Instead, use the SKErrorCode.PaymentCancelled enum.
For more information, see the StoreKit Changes for Swift page of the iOS 9.3 API Diffs and my canonical Q&A post Use of unresolved identifier when using StoreKit constants with iOS 9.3/Xcode 7.3.
