Sign In with Apple on Firebase - Invalid_Client - WHY?

[Jack McLaughlin]

Hey guys, 

I've followed this tutorial https://fireship.io/lessons/apple-signin-with-firebase-tutorial/ (Found on Youtube: https://www.youtube.com/watch?v=UafqYgRoIC0) but I am getting invalid_client on the Apple pop up. 

Google and Facebook logins are working as expected but Apple isn't. There aren't any errors so unsure on how I can begin debugging this. I've deleted everything in "Certificates, Identifiers & Profiles" within the Apple Developer website and started again to see if I did something wrong but I can't see that I have. 

Any advice or suggestions would be greatly appreciated. Thanks