I am just about to finish my tax return and this prompt comes up on the netfile page "Our records indicate you are not filing an income tax return for the first time with the Canada Revenue Agency. Please go back in your software and ensure you do not identify yourself as someone who is filing an income tax return for the first time and submit your tax return again.
If you are indeed filing a Canadian return for the first time you will need to click on "Print to Mail" and mail in your return. If you have filed a return previously with CRA then you will need to go back to Your Personal Information on the left-hand side and the second page will ask "Are you filing an income tax return with the CRA for the first time?" make sure you answer this correctly. Then you will be able to click on the "Review" section and continue.