Cal here from Shopify, thanks for reaching out to us. Is Paypal the only gateway on your store, currently? Have you completed the Paypal onboarding process?
I'd suggest contacting Paypal support directly about your Paypal account, to make sure that it is set up to receive payments. You can contact them directly here.
In order to process a sale through Paypal, your customers are redirected to the Paypal checkout (and taken away from Shopify), where it sounds like their payment is being rejected, before being returned to the Shopify checkout. Have you attempted to process a payment this way yourself, and can you replicate the customer's error?
Third-party gateways are maintained and developed by the gateway provider.
However, I'd suggest getting in touch with our live support team here who can investigate the active gateways on your store and make sure the setup was completed correctly.
No problem, I'm glad to help. I recorded a little video on how you can receive support using the link above.
Let me know if you still have any further difficulty contacting our live chat!