drush perm-grant gives "The drush command 'perm-grant' could not be found"

I am using drush -u 15 perm-grant --roles="site administrator" but the command returns:

The drush command 'perm-grant' could not be found.

any ideas about that? according this documentation the drush perm-grant is an available command. or maybe is some limitation of the user used to run drush?

the drush version is 5.4

thanks.