I talked with Customer Support and they said that the Pixel 3 and the Pixel 3 XL are on the approved list, but the Pixel 3a is not.
I do not see why Koodo has not added this phone to the list when it is a Google made phone and when previous models of the Pixel phones are supported.
Best answer by coltonbauman
Thanks for the reply Rob. Through some reddit-Fu I was actually able to get it working. If anyone is interested, here's what I did.
Turned off WiFi and data and went and turned on the WiFi calling toggle. It opened a web page but didn't load. It did however display this link that I then copied into my browser. I turned data on and hit refresh. Suddenly it brought me to the setup page. Followed the steps and it said it was now enabled. Turned WiFi back on, and suddenly the toggle works!
As someone who is tech savvy, it was a struggle for even me to get it to work when I knew it should. For the average person, it would have been a complete dead end to activate this, even though its doable.