Khankhattak
That's almost certainly carrier locked then. I don't know if Google can tell you who the carrier is.
You could try https://store.google.com/repair and see if provides clues as to the carrier. The carrier may or may not be able to help. As previously stated Verizon won't help.
It's been many years since I knew any 3rd party semi-legitimate phone unlockers, so I can't help there.
The official version of this is:
https://grapheneos.org/install/web#prerequisites
You need one of the officially supported devices. To make sure that the device can be unlocked to install GrapheneOS, avoid carrier variants of the devices. Carrier variants of Pixels use the same stock OS and firmware with a non-zero carrier id flashed onto the persist partition in the factory. The carrier id activates carrier-specific configuration in the stock OS including disabling carrier and bootloader unlocking. The carrier may be able to remotely disable this, but their support staff may not be aware and they probably won't do it. Get a carrier agnostic device to avoid the risk and potential hassle. If you CAN figure out a way to unlock a carrier device, it isn't a problem as GrapheneOS can just ignore the carrier id and the hardware is the same.
I think I've run out of ideas.