Usually "Validation Error" is encountered when there is an invalid format in the card details inputted in the checkout page. The expected format is as follows:

For Visa / Mastercard / JCB cards

• Card number: 16 numerical digits
• Exp Month and Year: MM/YY 
• CVN: 3 numerical digits

For American Express (AMEX) cards

• Card number: 15 numerical digits
• Exp Month and Year: MM/YY 
• CVN: 4 numerical digits