This article describes the situation when FortiAuthenticator is unable to send SMS messages.
The below message is appearing because the username is maybe too long:
To fix it, go to System -> Administration -> Replacement Message.
Choose the relevant SMS replacement message section.
It can be 'FortiToken Mobile Activation SMS Message'.
Then, remove some characters from the default replacement message to make it shorter, afterwards, attempt to resend the SMS to the intended user.