Kindly let us remind you that according to the Customer Agreement:
5.2.8. If an account was funded via debit or credit card, a card copy is required to process a withdrawal. The copy must contain the first 6 digits and the last 4 digits of the card number, cardholder’s name, expiry date, and cardholder’s signature.
This information is needed for security reasons, and it is a standard procedure for withdrawals via card.
Kindly note that CVC/CVV code on the backside of the card should be covered, though the sign in the particular field on the back of your card should be clearly seen since without it the card is considered invalid.
If you take a close look at the back of your credit card, you will probably see the "Not valid unless signed" caption.
Please, kindly take into consideration that merchants are prohibited to accept a credit/debit card unless it is signed.
To sign the card you need to place the signature manually on the backside of the card. Kindly note that you need to sign the card itself, not a piece of paper attached to it. You may use a pen or marker of any color.
You can see the example of the process here: