If you purchased a 2018 desktop edition, that should qualify to unlock previous years returns. But you will need to contact TurboTax Support to ask them to unlock the prior returns, since the unlock will not be automatic between the two different platforms.
Use the form at the link below to schedule a call back, or it may provide a number. Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Mon-Fri.
Where the form asks what product you are using, be sure to choose CD/download.
https://support.turbotax.intuit.com/contact
