Replying to this because in my case the process to install GrapheneOS was straightforward and I didn't have any problem. I use a relatively old laptop which has a USB port which makes connections disconnect from time to time (I also think I have a too low electric current flowing through it). Typically I can't even plug a medium size hard drive and I had disconnection issues with a hardware wallet and a drone. That being said, normally the cable delivered with the phone should work (white and thick cable in my case). If you have the same kind of problem I have, then try to avoid to put pressure on the cable during the install, because if the USB port on the laptop is old it may lead to disconnections. Do you have a device showing up with the command "adb devices"?