Are you a prepaid or postpaid customer?

You cannot send sms using wifi. You need to enable roaming to do that. Also sending text isn't free while traveling outside of Canada.

Are you using an iPhone? 

Are you trying to send regular text messages or send a message with a data based messaging app like Facebook messenger or whatsapp? 

I'm postpaid and using a Samsung android. I can use what's app, but wanted to actually text. I am on wifi with my flight mode turned on and my mobile data turned off.  

If you want to text (sms), then like Dinh said above you need to turn on and pay for roaming. Texting will not work on wifi.

Airplane mode will need to be turned off.

If you choose to do this, make sure you have Easy Roam enabled in your self serve

https://www.koodomobile.com/help/easy-roam