Fire Hi there, you have entered fastboot mode directly.
Now plug your phone into your Windows PC with the original USB-C to USB-C cable the came with your phone. Keep the phone in fastboot mode.
Next open Windows Update on your PC, and check for optional updates. There should be an optional update with "Android Bootloader Interface" in the name. That is the fastboot driver and what you want to install.
After that, you can proceed to the the unlock bootloader step.
If you have any further questions, i suggest joining the GrapheneOS chatroom on Matrix, where we'll be able to help guide you through the installation:
https://matrix.to/#/#grapheneos:grapheneos.org