This is a response from the Apple device operating system and is related to the adapter in use. Make sure to use an adapter certified by Apple.

If you have verified the adapter is certified by Apple and are still experiencing this issue test the MobilePay reader directly connected to a device without the adapter, and then test again with a different adapter on the device again. If the error persists when using the adapter, please contact Apple support.

If there are issues on a device without an adapter please see MobilePay card reader does not respond when swiping card.