The function is called using code similar to the following, from Drupal.
// Initialize the lock system.
require_once DRUPAL_ROOT . '/' . variable_get('lock_inc', 'includes/lock.inc');
_drupal_bootstrap_variables() is the only Drupal function that calls
That code could fail if the file whose name is contained in lock_inc doesn't exist, but in that case I would expect another warning/error message, or it doesn't contain a function called
I would check if there is code that sets
$conf['lock_inc'], and which value is used. I would also check if there is code calling