Here are possible reasons why you can’t restore your iPhone from an iTunes backup and what to do about them:
Corrupt backup – Your iTunes backup may have become corrupted. Use an alternative backup. If you don’t have one, use data recovery software. You can also try contacting Apple support.
Incompatible backup – Your iOS or iTunes may be outdated. Try updating first.
Bad internet – Your network may be disrupting the restore process. Make sure you have a strong connection before attempting to restore.