Do you know what carrier this phone is from? It looks like something in the phone is reporting that bootloader unlock is not possible on this phone due to the carrier not allowing it (e.g., as if it came from Verizon, who blocks bootloader unlocking).
But, if bootloader unlocking is blocked, you wouldn't have been able to install GrapheneOS in the first place, so this doesn't make sense to me.
Did you have to go through any extra steps to unlock the bootloader initially, or did you simply unlock it from the Developer Options menu in the stock OS as normal?