I have to copy-paste this as I have a further complication when installing GrapheneOS on my pixel 6 pro:
Quote " I am running into issues with unlocking the boot loader. I must be missing one step, but I can’t find which one. I have checked the thread https://discuss.grapheneos.org/d/15198-bootloader-unable-to-unlock and also other pages without any success

So far I have:

Turned developper mode on (multiple click on Settings/About phone/Build number)
Toggled ON “OEM unlocking (in Settings/System/Developper options)
Restarted the device with "volume down” pressed
The device is restarting correctly in “Fastboot mode”
but this is where I run into my issue:
In Fastboot mode, the "Device State" is marked “locked” in green
When trying to toggle the menu with the down button I get the following options:

Start
Restart Bootloader
Recovery Mode
Rescue Mode
BarCode
Power off
But I cannot see any unlock boot loader menu.

Any idea on what I may have missed?" Unquote. Exactly my situation except the solution in the comment won't work for me.

So I did what this thread above was saying (https://discuss.grapheneos.org/d/15752-issues-unlocking-the-bootloader) but there seems to be an issue with the phone connection, usb connection fails me or I don't know what else it may be. After I press 'Unlock bootloder' my phone is detected but then the message says in red: "Error: Failed to execute 'open' on 'USBDevice': Access denied." It's desparate. Any help?
Note: I am using Ubuntou 24.04 but I tried Windows and my pixel 6a as well (with Graphene on it)! The same result everywhere...