To add in what @itsjpb said, you will need to connect the phone to the internet as well (either WiFi or cellular) so that it can verify it's not stolen (I assume) and allow you to enable OEM Unlocking. Otherwise, the option will be disabled until then. That was my experience at least.
Once you go through the initial setup of stock Android, and it's online to verify that, toggle the OEM Unlocking. Once that is enabled, you're free to reboot into fastboot, unlock the bootloader, and flash GrapheneOS. You do not need to sign in to an account or activate cellular service prior to, unless WiFi is not available and the setting is blocked.