My guess is the prep.17 didn't get copied to C:\Lacerte\17tax

You can download the file from any of the years. Go to My Account: https://myproconnect.intuit.com/#/accounts/dashboard and click Previous Years below DOWNLOAD PREP FILE. Then select the year 2017 and be sure to download it to C:\Lacerte\17tax It should show PREP.17 in the bottom left of your browser. Right click that and select "Show in Folder". It is likely in downloads. Highlight it and use "CTRL + C" to copy. Navigate to C:\Lacerte\17tax and use "CTRL + V" to paste it. Now return to the Dashboard at My Account and Download 17 it should install now.

Another guess is that the loader is on your computer. Try C:\Program Files (x86)\Common Files\Lacerte Shared\w17setup.exe OR hit windows key and type 2017 Lacerte Web Setup