How come I can't port in my own number from Rogers and replace my current trial Fizz number ?

Fairly sure that's the way it is with all companies. Once the card's activated to an account, you can't re-activate it to another one...

I think you can only port to inactive sims. Don't believe there's anything you can do now, unless you get a new sim

To my knowledge porting can be only done on an inactive sim. Try contacting their help on facebook messenger, or live chat on their website. :)

You can probably have them deactivate the sim, and then you can port the number to a new plan on the sim. If you joined before April 12, you'll lose the introductory pricing over the long term though.

According to Fizz, the line you want to transfer must be active with your old provider and must be done at the time of SIM activation.