Fatal error when activiating JS Help Desk Plugin

When I attempt to activate the JS Help Desk Plugin, I get the following error:

Fatal error: Uncaught Error: Call to a member function add_cap() on null in /home/nwpsafeu/public_html/site/wp-content/plugins/js-support-ticket/includes/activation.php:21 Stack trace: #0 /home/nwpsafeu/public_html/site/wp-content/plugins/js-support-ticket/includes/activation.php(13): JSSTactivation::addCapabilites() #1 /home/nwpsafeu/public_html/site/wp-content/plugins/js-support-ticket/js-support-ticket.php(117): JSSTactivation::jssupportticket_activate() #2 /home/nwpsafeu/public_html/site/wp-includes/class-wp-hook.php(288): jssupportticket->jssupportticket_activate('') #3 /home/nwpsafeu/public_html/site/wp-includes/class-wp-hook.php(312): WP_Hook->apply_filters('', Array) #4 /home/nwpsafeu/public_html/site/wp-includes/plugin.php(478): WP_Hook->do_action(Array) #5 /home/nwpsafeu/public_html/site/wp-admin/plugins.php(177): do_action('activate_js-sup...') #6 {main} thrown in /home/nwpsafeu/public_html/site/wp-content/plugins/js-support-ticket/includes/activation.php on line 21

PHP ERROR:array(4) {
  ["type"]=>
  int(1)
  ["message"]=>
  string(873) "Uncaught Error: Call to a member function add_cap() on null in /home/nwpsafeu/public_html/site/wp-content/plugins/js-support-ticket/includes/activation.php:21
Stack trace:
#0 /home/nwpsafeu/public_html/site/wp-content/plugins/js-support-ticket/includes/activation.php(13): JSSTactivation::addCapabilites()
#1 /home/nwpsafeu/public_html/site/wp-content/plugins/js-support-ticket/js-support-ticket.php(117): JSSTactivation::jssupportticket_activate()
#2 /home/nwpsafeu/public_html/site/wp-includes/class-wp-hook.php(288): jssupportticket->jssupportticket_activate('')
#3 /home/nwpsafeu/public_html/site/wp-includes/class-wp-hook.php(312): WP_Hook->apply_filters('', Array)
#4 /home/nwpsafeu/public_html/site/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#5 /home/nwpsafeu/public_html/site/wp-admin/plugins.php(177): do_action('activate_js-sup...')
#6 {main}
  thrown"
  ["file"]=>
  string(92) "/home/nwpsafeu/public_html/site/wp-content/plugins/js-support-ticket/includes/activation.php"
  ["line"]=>
  int(21)
}