If your issue is that they're able to get to the payment confirmation on PayPal, and then it gives the error after they've attempted to submit payment, you'll want to check some of the following elements in your PayPal setup:
Make sure your PayPal account is a Premier or Business type:
Log in to Paypal
Click the Profile option at the top
Choose Profile and Settings
Choose My Settings to the left
The Account Type will be listed to the right
Accept PayPal payments from all your online stores:
Click the Profile link in the top right corner
Click Profile & Settings
Click My Selling Tools in the left corner
Click Update next to API access
Click Enable Express Checkout, under Accept payments from your online stores before setting up APIs
Put a checkmark next to Accept PayPal payments before API permissions or credentials are set up (If Block payments from uncredentialled shopping carts is selected, the error will occur)
Make sure that a valid Tax ID number is on file:
Click the Profile link in the top right corner
Click Profile & Settings
Click My business info in the left column
Click to add either a Social Security Number, Individual Taxpayer Identification Number, or Employer Identification Number
Make sure that the email address is verified:
Click the Profile link in the top right corner
Click Profile & Settings
Click My business info in the left column
Click Edit next to the address
Click Confirm this email to receive a confirmation email
Click the Confirm your email button in the email that is received
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