Q: my 5th gen. ipod will not restore, when I try it says it can't because of an unknown error.

Is your computer running iTunes a Mac or Windows PC?  If Windows, what version?

Try shutting down the computer and disconnecting all USB devices.  You can leave standard USB keyboard and mouse connected, if used.  Since there may be a data corruption issue on the iPod, put the iPod into Disk Mode

Putting iPod into Disk Mode - Apple Support

Start up computer, run iTunes, and connect iPod.  If iTunes picks it up, it may prompt you to do a Restore with a message about "recovery."  Tell iTunes to proceed with Restore.

Did you try disconnecting other USB devices and connecting the iPod (by itself) in Disk Mode?

It should go into Disk Mode, but if you can't get that to happen, try connecting the iPod normally (to a direct USB port on the computer) with other USB devices disconnected.  Does the message about corrupted files and needing to do a Restore still appear?  If you do a Restore, do you still get that error?