"Template "default.html.twig" is not defined." after configuring plugin

I have installed "Admin Addon User Manager v2.1.5", Created a group and assigned a user. When I try to 'login as' it returns to /admin and displays an error "Template "default.html.twig" is not defined."
Is this related to the plugin or do I need to change the template files?
Twig/Twig.php throw new \RuntimeException($error_msg, 400, $e);