Error 51 iPhone 7 after restoring

it sounds like the IPSW file format is corrupted. the customer might have run a beta or jailbroken version of iOS which can usually be 1 of many reason for error 51

delete the existing saved IPSW in the iTunes directory

for Mac

1. Launch Finder
2. Click Go from the menu bar
3. Scroll down and click ‘Go to Folder’
4. Type in “~/Library” and click Go
5. Click and open the iTunes folder
6. Open the iPhone Software update
7. Delete the.IPSW files

for Windows

1. Go to C:\Users\Your Computer Name\AppData\Roaming\iTunes\iPhone Software Update
2. Find the.IPSW file and delete or you can press CTRL +A (control all) and hit the delete on your keyboard.
3. Update the iPhone again.
4. trying an update after this procedure should force iTunes to download a new file format
   

try downloading a fresh IPSW image currently Apple only sign 11.4.1 and Proceed to update the iPhone

hope this works if that doesn’t work. you might wanna give FoneDog toolkit IOS system recovery a try

cheers