An actual human being wrote:

The payment is not going through because the Internal Security model of PayPal has reviewed the payment and has prevented it from completing.

There are certain reasons why a specific transaction cannot be completed: (1) There’s a "spike" on your payment history, (2) a change in your spending patterns or (3) it could be anything that the system might find unusual with that particular transaction; e.g. if the account was being accessed from a different location.

The security of your financial and personal information is our first priority. If your account is accessed from an unsecured locations and we suspect that it might show unauthorized activity, we must act to protect your data.

Nevertheless, here are the options for you to be able to send your payment:
•Use a different funding source.
•Try to send the payment at a later time.
•Ask for an invoice from the seller.