The package for FP3 has been replaced, you can download it again: https://github.com/iodeOS/ota/releases/download/v4-FP3/iode-4.6-20231107-FP3-fastboot.zip
The problem was an incorrect version of fastboot. We have embedded the required fastboot executables in the package, so the installation should now work regardless of the already installed fastboot version.