The license file is tied to the serial number generated when you first boot the VM. Do not delete the VM before installing the license.
: Extract the files and import the virtual appliance into your hypervisor. Important Trial Limitations Starting with FortiOS 7.2.1 fortigate-vm 7.0 download free
FortiGate 7.0 offers a specific evaluation period that differs slightly from newer versions: The license file is tied to the serial