Welcome to the community!
Sorry to hear you're having issues with sending text messages. What phone are you using? Have you verified that your plan includes text messaging? As far as I am aware, the Tablet plan does not include text messaging. Are the messages you're trying to send SMS or MMS? If you were trying to send MMS, had you verified that your phone's data setting was enabled?
You might consider verifying your phone has the proper short message service centre (SMSC) number. Fido's SMSC is +1-514-993-1123 (see here). In addition, you might also consider verifying whether your phone has the proper MMS settings. You can verify those settings here.
Hope this helps 😀