Did you use TurboTax Free Online for your 2018 tax return? If so, you won't be able to find it at the bottom of the return manager. The TRA (Tax return access) service was not available to free customers before 2019 (unless they added Plus to their account).
However, you can still download the PDF of your 2018 return, but you'll have to login to 2018 to do so. Once you are there, open your return, then select File > Print Your Tax Return from the bottom of the left side menu. You can login to TurboTax Online 2018 here: https://ww1.2018.turbotaxonline.intuit.ca/
