Schedule package help

Hello,

I’ve found a page which is just what I’m looking for to for my website. Its called phpScheduleIt https://php.brickhost.com and it has some sort of plugin for WordPress. But it doesn’t work :(. I get an error which says:

Fatal error: Call to a member function add_query_var() on a non-object in /home/guildfor/public_html/wp-includes/rewrite.php on line 47

I don’t want to touch anything in the core of WordPress in case something goes wrong. Could someone help me and find out what this error means and how it can be fixed. Thanks