This could be because the User does not have texting enabled or the number is stuck in a loop.
To fix this, remove the Users number by clicking the red X to delete it and re add the number again.
You should then get the code and now be enabled to go in and checkoff the notifications the user would like to get text notifications from.
Note: Each phone provider has there own way of blocking these types of notifications, so issues like this are often provider specific. For At&T, on their website they have a support article that explains how to manage blocked notifications. Here is the link: