Payment for the desktop edition does not unlock the online editions. You have to upgrade to the Plus edition on the 2018 online tax return and then pay for the upgrade to access the prior year online tax returns. As explained in the answer to your question.
If you do not need the desktop edition you paid for then you can request a refund using this TurboTax website - https://support.turbotax.intuit.com/forms/refund-request.jsp
