This is the most common error from RevenueCat that can also lead to unexplained App Store rejections.
Unfortunately, this error occurs when Apple's /verifyReceipt endpoint is down and unable to verify purchases. In sandbox, these errors can be fairly common since the sandbox environment is not as robust as production. In production, expect these errors less often. If your app is working in sandbox mode while testing on your own devices, this is likely an issue with the App Store.
App Review
This issue seems to affect new apps (apps that have never been released on the App Store) during their first review more than apps that have previously been released on the App Store.
Generally, the best course of action is usually to re-submit your binary or try to explain the situation to the reviewer. (Some developers have reported that they had to submit their binary over ten times before it started working as expected for the reviewer!)
Just to clarify, there's nothing you as a developer or RevenueCat can do to resolve these errors except retrying later.
Outages
There are momentary outages in sandbox from time-to-time, but occasionally outages can last for days at a time. We'll do our best to flag major outages on our status page at status.revenuecat.com.
Check out our docs for more information about this error.
