Guestbook - Error: One of the input...

Okay, I've been hanging out in the other posts looking for an answer that will fix my issue. I'm using 2.3.1 (I don't know where to go to upgrade), I'm not seeming to find it on the web... but the error of "One of the input fields does not seem to be valid." has been plaguing me for a couple of hours now. I've tried some of the fixes that I'd seen in some other threads, but they don't seem to fix my issue. Someone please help me. I swear I'm not usually this dumb. The guestbook is located at:
www.simonsangels.org/guestbook

Thank you in advance

You upgrade by downloading 2.4.1 from this site and following the instructions found in this forum.