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:
Unable to send FTM activation message to xxx. FortiGuard server failed sending SMS because message is too long.
Step 1: Go to System > Administration > Replacement Message.
Step 2: Choose the relevant SMS replacement message section. It can be ‘FortiToken Mobile Activation SMS Message’.
Step 3: Remove some characters from the default replacement message to make it shorter.
Step 4: Attempt to resend the SMS to the intended user.