Can you use a Digital product Key (from Amazon) on the website? I do not see an option to enter my product code anywhere on the website.

The TurboTax online edition do not require a license code.  Only the 2019 TurboTax software installed on a personal computer requires a license code.

NO ... you bought the Downloaded version and the licence code is for that program only.  The ONLINE version is a separate program and the code will not work on it.  

Now ... if you started with the ONLINE version by mistake and would like to switch to the program  you bought to finish you can ...    

1. Save your online return as a tax data file.
2. Sign out of TurboTax Online.
3. Install your 2019 TurboTax software. 
   • Important:If your return includes a state, make sure you also install the corresponding state program(s), otherwise you'll get Error 539.
4. Once installed, choose File > Open Tax Return in the software program.
5. Browse to where you saved your .tax2019 file and click Open.
6. If you get an error, delete your tax data file and repeat these steps from the beginning.
7. (Optional) Once you've confirmed that your online return opens properly in the software, you can clear your online return.

Thanks for the clarification, wish I would have known before buying :(

Why ?   The Downloaded version is cheaper  AND  superior.   Once you use it most will never go back. 

Don't you have a computer to install it on?  You need a full Windows or Mac computer.