The CRA will present you with NETFILE code 16 when it believes that you've never filed a Canadian tax return,. To remove this error, refer to the following list of instructions:
If you're still unable to resolve the error, Contact the Canada Revenue Agency (CRA) to make sure your information matches what the CRA has on record.
With the online version, in the Let's get the rest of your personal info step, there is no setting to say that I have filed a tax return previously. When I first created my account, I believe I accidentally selected that I was a first time filer.
@funtimes4 It has changed in 2023. Now you have to go to My Info > [Name]'s Canadian Citizenship.