therein lies your problem. this is a normal problem with the host installed versions of phpbb.
they do not use the same files you get here.
your best bet is to download the correct files from here ( download button above ). then replace all the files you have with the ones in the zip file you downloaded (except for the config.php file, be sure to save the one that you already have.
then, run the database update .
basically follow this:
https://www.phpbb.com/support/docs/en/3 ... tallation/