So you seem to be finding that your (I assume platform-tools) fastboot finds the phone
but the script’s ${FASTBOOT_BIN} does not.
I wonder if Search ${FASTBOOT_BIN} gives you any clue.
Also Script ./flash_FP6_factory.sh uses shipped fastboot binary indirectly refers.
Is there any possibility that frp is involved? That is any remaining Google account, password or PIN.