Fatal Error with ActiveCampaign API

Installed and activated this plugin, but when I enter my ActiveCampaign API info I get a the following error messages:

Warning: require_once(ActiveCampaignAPI/ActiveCampaign.class.php): failed to open stream: No such file or directory in /home/softpres/public_html/wp-content/plugins/wp-subscribe-form/includes/sfba-subscription-page.php on line 227

Fatal error: require_once(): Failed opening required ‘ActiveCampaignAPI/ActiveCampaign.class.php’ (include_path=’.:/usr/local/php56/pear’) in /home/softpres/public_html/wp-content/plugins/wp-subscribe-form/includes/sfba-subscription-page.php on line 227