The web installer is not in coherence with the install method for this device, why it fails.
better switch to the new “install-script” method
for the FP6 install, adb is only used to reboot the device in bootloader mode,
you can do that using the buttons…
BUT the error can also signify that your cable is not data capable.
also, if you use windows, check [Solved] Windows does not see my phone in Fastboot mode