I purchased Turbo Tax through Office Depot. Can I use the proof of purchase to complete filing online or do I need to fill out my return in the downloaded version?

[VolvoGirl]

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)