iPhone 3GS error, no backend available when libusb is alr installed.

I am trying to downgrade my iPhone 3GS 16GB new bootrom from 4.1 to 3.1.3 using an exploit, it is telling me this (legacy iOS kit)

*If you get the error "No backend available," install libusb in Homebrew/MacPorts

[Log] Done!

* This may take several tries, It can fail a lot with "Operation timed out" exto If it fails, try to unplug and replug your device then run the script again.

• You may also need to force restart the device and re-enter DFU mode before retrying

• To retry, just go back to other utilities → Install alloc8 Exploit