Hi Kelly-
From what you’re describing, it sounds like you’re experiencing a JavaScript error while adding a new menu item.
I checked out your site, and it looks like you’re running a pretty old version of your Theme, Grid One. You’re currently running version 1.20.2 which was released December 2016, and that version hasn’t been tested with up-to-date versions of WordPress.
Try updating your Theme first, and see if the error persists once you’re running the most recent version of Grid One.