I tested in the sandbox without issues and then switched to production and received this error...
Payment error: PayPal error: An unknown error occurred. (UNKNOWN_ERROR)
I contacted PayPal and they told me it was...
Error code: 10561 "Invalid Data" "There's an error with this transaction. Please enter a complete billing address."
and that I needed to pass the full billing address for transactions to be approved.
I am using nopCommerce 3.9 and have my REST API setup correctly with PayPal and I honestly don't see any issues on the nopCommerce end. Everything looks normal and my sandbox test order worked fine.
Anyone run into this before? Any help would be greatly appreciated.