Very new to Shopify and one of my first sales was by Paypal. I had notification of the sale and confirmation as follows
A $227.00 AUD payment was processed on PayPal Express Checkout.
Hank here from Shopify.
I looked up that error message and it seems to be related to PayPal directly. The most common answer I've seen to that query is related to the subject. It states "if so, the email address in 'SUBJECT' does not belong to an existing PayPal account".
I also found the following for you, "this error is encountered after returning to the cart following the PayPal payment entry". If you are noticing that the payment seems to be successful that is great, but I would recommend reaching out to PayPal support to ensure that there is nothing else that is relating to that, and to confirm it was just a once-off error.
If you do have any further questions, let me know and I'll happily assist you further.
All the best,