Usually when someone gets an error when they are texting into the shortcode, there are a couple of things to try.
Check first to see if they entered the correct keyword.
If they are entering in the correct keyword, and it is still returning the error, then the most likely reason is because they have shortcodes blocked on their carrier or they have a mobile phone from an international carrier. Not to worry, it is precisely for this reason that Mosio provides a virtual number as well. If your attendees cannot get through using the shortcode, they will be able to with the Virtual Number.