Download the update again, it may be corrupted.
You could calculate the checksum of the update to be sure, read this, Trying to update from 6.3 to 7.0 (but no WIFI, and no mobile data) - #4 by george
there is an app on Aurora for this purpose, https://play.google.com/store/apps/details?id=com.raspreet.checksumcalculator
if it persists you can do a factory reset and repeat the process