If a user complains about transaction error/s, there might be a couple of reasons why. The most likely cause for this is the billing address entered in the checkout doesn’t match the card or the CVV not matching.
The possible solutions are:
Ensure that the address entered matches the card
They could use the Paypal payment option, and then pay with a credit card using the guest user (you don’t need a PayPal account).
If they're using a corporate card outside of US, ask them to use a personal card.
Saved Replies:
Transaction Error
Paypal Credit Card Payment Method