Did you get the CD installed ok? Sorry you can't apply the Desktop purchase to the online version. They are separate things and don't talk or sync to each other. You can move your online return to the Desktop program without paying for online so you don't need to start over.
First download the .tax2018 file from the Online version,
https://ttlc.intuit.com/replies/3300420
Then see, How to Switch to Desktop,
https://ttlc.intuit.com/replies/3302260
After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - New)
YOU DON'T HAVE TO PAY FOR THE PROGRAM TWICE! If you've done much or all of your work using the online version of TurboTax but have also purchased the disk and have an unlock code, you can do the following.
Go to TurboTax and use your unlock code to download the more up-to-date version. Install the program, the file will likely be in you download folder. After the install, open the program and get the latest updates when the program opens. Do not save your tax file yet!
In the online TurboTax, on the left side of the page you can go to the Tools tab and choose "Save a Copy". Save your copy of your online work somewhere you will be able to locate it, but mine went into my Downloads Folder. In your local device, open Turbo Tax. Click on the 'File' tab, select 'Open File' and click on the file you saved from online TT. Your work is now in your purchased product and you do not have to pay for the online version.