PayPal error 10004

First Woocommerce store. I’m using the “PayPal Checkout” extension. I turned on Sandbox mode and tested it by using PayPal checkout (signed in into my PayPal account as if I were the customer and authorized the purchase). It worked great.

Then I tested it as if the customer is paying with a credit card. I generated a fake credit card number from the PayPal developer website. The transaction does not go through, and I get the following error:

PayPal error (10004): An error (10004) occurred while processing your PayPal payment. Please contact the store owner for assistance.

I turned on the log feature. There is too much to paste here, but at the bottom is the following:

[TOKEN] => EC-69D95753A5556104D
[SUCCESSPAGEREDIRECTREQUESTED] => false
[TIMESTAMP] => 2019-03-13T11:56:22Z
[CORRELATIONID] => 87cf7830672a7
[ACK] => Failure
[VERSION] => 120.0
[BUILD] => 52123114
[L_ERRORCODE0] => 10004
[L_SHORTMESSAGE0] => Internal Error
[L_LONGMESSAGE0] => Transaction refused because of an invalid argument. See additional error messages for details.
[L_SEVERITYCODE0] => Error

Can anyone please point me in the right direction of how I can resolve this? Thank you in advance.