If this message appears on your mobile device, try restarting the phone forcefully, remove apps that you are not using, especially the free ones, which may be malicious. Then open your web browser and clear your cache.
If instructions above do not work, contact a tech-savvy friend and have him/her help you get rip of the fake pop-up message.
But, never send money or iTunes gift card numbers to the scammers or cybercriminals behind the fake pop-up message.