Have you already filed the return using NETFILE? If you have, then you won't be able to make changes to your return unless you are doing a REFILE. This TurboTax FAQ has instructions on how to use REFILE: I've filed my tax return. Can I still make changes?
If you haven’t filed your return yet, you can go to the T4A Slips page by clicking on Find (magnifying glass) in the upper-right corner of your screen. In the Find window, type in “T4A” and select “T4A Pension, Retirement, Annuity” from the list. Click the “Go” button. Click “Enter New T4A for [Your Name]” to add a new T4A.
If you are still not able to resolve this issue, please contact our phone support team. You can use the form on this page to get the right phone number to call: TurboTax Contact Us