Firmware upgrade without license

https://docs.fortinet.com/document/fortigate/7.6.5/administration-guide/299518/how-the-fortigate-firmware-license-works 

https://docs.fortinet.com/document/fortigate/7.4.0/new-features/299518/prevent-fortigates-with-an-expired-support-contract-from-upgrading-to-a-major-or-minor-firmware-release 

depending on your support contract expiration date, i asume somewhere before March 25, 2025, you are not able to upgrade to 7.6

one option would be to go back to a version prior to 7.4.2 when this was enforced and upgrade directly to 7.6

How did you download 7.6.5 then? Why would you expect to be able to upgrade this without a license? If this was a demo unit or you happen to work for a partner, work with your Fortinet account team.

In order to upgrade to 7.6.x you will need an active subscription.  Minimum of Forticare support.  Whilst you may be able to upgrade within the 7.4.x release, this does not cover upgrades to a new major version.

https://docs.fortinet.com/document/fortigate/7.4.0/new-features/320693/automatic-firmware-upgrades-for-fortigate-appliances-with-invalid-support-contracts-or-that-have-reached-eoes-7-4-8