Factors contributing to iPhone unknown errors when updating or restoring with iTunes can generally be summarized into 2 categories: hardware and software.
If it is the hardware issue that causes such errors, just visit the Apple store to ask for help.
If it is the software issue that causes such iPhone errors, try 2 methods listed below.
1. Make a backup of the iPhone with iTunes or iCloud. After that, put the iPhone into recovery mode and connect to iTunes to restore it;
2. If you don't mind using a technical tool, you might as well try Joyoshare UltFix. It can rectify various iPhone unknown errors without data loss.