"This device is not eligible for the requested build: Also sometimes displayed as an error 3194. If you receive this error, update to the latest version of iTunes. If you do have the latest version of iTunes, this can occur when you attempt to restore using an older .ipsw file. Try restoring in a new user to ensure that iTunes downloads a new IPSW. 3rd party security software or router security settings can also cause this issue, to resolve please follow Troubleshooting security software issues. Downgrading to a previous version of iOS is not supported. If you have installed software to perform unauthorized modifications to your iOS device, that software may have redirected connections to the update server (gs.apple.com) within the Hosts file. For steps to edit the Hosts file and allow iTunes to communicate with the update server, see iTunes: Troubleshooting iTunes Store on your computer, iPhone, iPad, or iPod; follow steps under the heading Blocked by configuration (Mac OS X / Windows) > Rebuild network information > The hosts file may also be blocking the iTunes Store."
Also, if you "successfully" hacked the phone to downgrade the iOS in the past, none of these techniques will work.