From Joomla! Documentation

When registration is done with administrator activation you may get this error:

Fatal error: Call to a member function where() on a non-object in
 ROOT/components/com_users/models/registration.php on line 109 

Where ROOT is the path to your Joomla files.

To solve this, change line 108 of ROOT/components/com_users/models/registration.php





(There should be an additional ) at the end of the line.)