Endpoint Protection 12.1 Activate License problem

Hi, I have some problem about activate license process on symantec endpoint protection manager.

I had import license file ***.slf  (This license file never to activate since I got this file more than 1 month), then it's show "This license has reached its expiration date and is no longer valid. To purchase a new license, Contact your preferred reseller. For more information, visit the Symantec Licensing Information Center"

How can I renew and activate my license key?

Thanks