Apple is not supporting the earlier models of iPhone with the later versions of iTunes.
iTunes provided this error message "iTunes could not connect to the iPod touch because an invalid response was received from the device." using my Generation 1 iPhone (2007) and iTunes version 12.6.0.100
There are two solutions for this, either find an Apple computer running an earlier version of iTunes, or find a PC version of iTunes that is earlier than version 12.
I found iTunes Version 8.0.2.20 for the PC (64bit) and that worked fine on my Win8.1 PC.
Caution - If you try to reload iPhone firmware holding down the power plus home button, and keeping the home button pressed, but you dont have a working iTunes - you risk "bricking" the iPhone until you can reload the firmware with an older version of iTunes.
The FileHorse website provides many older versions of iTunes for the PC.