Welcome to the community!
Is your phone from Fido? Are you trying to send a SMS or MMS? If you're trying to send MMS, have you verified your data is enabled? Have you verified that your phone's APN includes the proper MMS settings? You can verify those settings here. Have you verified your Short Message Service Center (SMSC) number? It should be +1-514-993-1123.
Hope this helps 😀