Cannot reactivate w3 total cache plugin

public/wp-content/db.php could not be created, please run following command:
chmod 777 public/wp-content/db.php

That’s what keeps popping up when I reactivate the plugin. I’ve already done these:

chown nobody:nobody wp-content -R
chmod 777 wp-content
chmod 777 wp-content/db.php
rm -f db.php //removed it

I don’t know what else which might be causing the problem.