Hm, it's not very clear to me if you get this before or after restoring the site.
If this happens before restoring a site, using the semi-auto restoration may be best. Please extract your backup archive on your local PC using Akeeba eXtract Wizard. Then upload all files to your site and visit http://www.example.com/installation/index.php, where www.example.com is the domain name of your site. It's best to remove the .htaccess and php.ini files before trying to run the restoration script. In this case you should also remove the backup archive and kickstart.php before uploading the files, just in case you're running out of disk space.
If you receive the error after restoring your site, when trying to access your restored site, most likely you have a partial backup archive, a partial extraction or some files were simply missing from the backup. In this case it's best to check the backup log file for any lines beginning with WARNING which may be talking about unreadable files. These are the files which have not been backed up because their permissions did not allow them to be read.
Nicholas K. Dionysopoulos
Lead Developer and Director
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