No error message

The form works fine when I type in correct information, but when I miss anything and an error message should appear alerting me to that, it doesn’t. Instead, I am redirected to a “page not found” page. Can you help me figure out why isn’t not giving the error alerts?

Here’s the site form: http://alsocialstudiesonline.org/general-presenters

Thanks,
Jaimee

https://ww.wp.xz.cn/plugins/si-contact-form/