Make sure you're using an SKProduct that has been retrieved via a StoreKit query immediately prior to making the purchase. Do not cache SKProducts, otherwise some users will get this error.
Aidano101
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There's no need for sarcasm. I've resolved sandbox issues this way when nothing else works – including the obvious step of deleting the app completely before installing a new build, which you're implying that the poster has somehow failed to bother trying.