Before beginning an online tax return, TurboTax forces you to select items that apply to your tax return so that it can show you the price and start you out with a version that will meet the selected needs. Whether intentional or not, the selections do not include one for retirement income, forcing one to select something else. To start with the free edition you must select either that you have a job and receive a W-2, that you paid rent or that you have children or dependents. Selecting anything else will start you off with a paid version from which you can downgrade only by clearing and starting over. Without a specific selection for retirement income one might be tempted to just select the option to maximize deductions and tax credits, which might seem innocuous, but doing so this will trigger the use of TurboTax Deluxe which is not free.
To get the free version, simplest is to just select the item that indicates that you have a job, but then don't enter any W-2. If you do ultimately enter anything that truly requires a paid version of TurboTax, TurboTax will prompt you to upgrade.
