Normally the way to qualify to unlock a 2017 online return is to purchase a 2018 product. Purchase of a 2018 online product (PLUS is the minimum) in the same account would unlock it automatically. While a 2018 desktop software purchase also qualifies to unlock a 2017 online return, the unlocking would not be automatic. Customer Support has to unlock the online returns in that case.
I realize you said you purchased 2017 desktop software. You can discuss it with Customer Support. I'm not sure what they will do. Technically, they may say it has to be a 2018 software purchase to unlock past-year online returns, but it can't hurt to try. Tell them you already had to spend a lot of money just to buy the 2017 product.
You can use the form at the link below to schedule a call back, or it may provide a number. Hours are 5AM-9PM Pacific (8AM-12 Midnight Eastern) 7 days/week.
Where the form asks what product you are using, be sure in the dropdown menu to indicate "CD/download" and the proper operating system, then indicate which Edition.
https://support.turbotax.intuit.com/contact
