Not working with HeadwayThemes?

Hi, I just recently downloaded this to try to have it work as the invoicing system for my wordpress site. The site I installed it on uses Headway Themes (version 3.6), is updated to WordPress 3.8, but when I installed this plugin, and then go to save the changes made in settings, it redirects me to my error page. Any ideas how to fix this issue?

Site Link

http://wordpress.org/plugins/wp-invoice/