The default Joomla template (Protostar) comes with predefined positions such as position-1, position-2, position-1 etc, however not every single one is defined in the index.php of the template folder and therefore if you assign a module to one of these undefined positions, it won't be displayed.
Assign your module to position-7 which is a pre-defined position on the right hand side of your template. Other modules such the main menu and login also use this position.