Two points - you need the -R *before* the 664 and even if you're in the
folder you want to change, you need to tell it what to chmod, so you
need to cd to the directory and 'chmod -R 664 .' or 'chmod 664 *' should
work as well if there are no sub-folders - you don't need the 'sudo' if
the files are all owned by the logged in user, presumably yourself.