I experienced an issue flashing back to the standard Google Pixel (stock) firmware. I resolved the issue and am writing this post to share the knowledge.
The device is a Pixel 7 Pro and was running the 01 build of GrapheneOS.
The installation back to stock firmware was performed on a Linux system using command line installation.
When the device with GOS installed is first booted into the bootloader interface it is not recognised in terminal.
The fix is to leave the device connected to the computer and "reboot to bootloader".
When the device is in the bootloader interface, using the volume keys select "reboot to bootloader", then use the power button to execute the command. The device will reboot to bootloader and you should be good to go.