Problem with reseting icloud password

Please a sister gave me iphone 6 and I had a hardware issue that required system restore. After the restore the phone is asking for activation with icloud. Unfortunately she does not remember the password to the account. She has tried several times to reset it but when she enters the master card number required, she keeps on receiving a message that it is wrong. Please is there any way this problem could be solved? Please I really need your help.

Thank You