An issue arose when I was trying to restore an old iPhone backup. iTunes could not restore the iPhone because the backup was corrupt or not compatible with the iPhone being restored. The issue was I had factory reset my old device and now I was receiving this. I thought it was all over.
My searches online led me to a software called Phone Rescue and after scanning my backup it found everything then tried to charge me $50 for it. I looked back at the screen with my iPhone backup on it and it didn't have the corrupt mark next to the backup.
I thought about it and I hadn't updated my iPhone software before the backup and my new phone didn't match.
I found a site that allows you to download firmware. Open Phone Rescue and look next to the backup and download the version that's next to the backup name.
Once its downloaded open iTunes and shift-click (PC) or option-click (Mac) the Restore button and find your download.
If you see Error 3194, Error 17, or 'This device isn't eligible for the requested build' in iTunes then you will need to update your iTunes to the latest version.
Once its restored to the old version of iOS then you can restore your backup of your iPhone.
I hope this helped as it saved me from losing everything. Feel free to buy me a coffee if it helped, shown in the footer.