I filed my tax but at the end it's giving me a CRA authentication failed error which I can't get around. I already paid $20.

Do you have a NETFILE code entered in your return? If yes, it could be the error you are receiving.

The NETFILE code is optional. This means that even if you remove it completely, there will be no impact on your return. If you made a mistake when you entered it, here's how and where to correct it.

In TurboTax Online, you can correct it or delete it completely. When you click on CRA NETFILE, on the Verification of your personal information page, just before the conditions for NETFILE, there's a place to enter your code. Simply delete or correct it. 

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