Releasing the license and Re-activating the license

Did you try to do the re-activation before the upgrade? It is required. See K7727: License activation may be required before a software upgrade for the BIG-IP or Enterprise Manager system.

"The service check date in the BIG-IP license is updated each time the license is reactivated, if there is an active service contract for this BIG-IP system"

You need to have active support contract. You can check the contract status with serial number of the BIG-IP from here: Product Information Request. Enter your e-mail address and the serial(s). After some minutes you should get information mail of the appliances with support level and validity dates, etc.

If you did these and you have a valid support contract, you can always raise a ticket to F5 support.

Releasing the license applies to VE appliances, where you can upgrade the hardware and move the existing license to the new hw platform. In V13.1 new features:

"New in 13.1.0 :: VE Reusing Licenses You can now revoke a license from a running instance of BIG-IP VE, and then use that license on another BIG-IP VE."

Before 13.1 the release and reuse could only be done with F5 support.