Turbo Tax won't let me download my 2016 tax return. Any application I use to open the file once its downloaded, says it can't open on my computer. Help?

Are you downloading the PDF ?   Do you have a PDF reader like Adobe ?  What is the file extension if it is not a PDF ?   What browser are you using ? 

You would need to download the .pdf version, not the .tax file.  You can only open the .tax file with a desktop version of TurboTax.

Also, if you are using Microsoft Edge to download the .pdf file there is a current compatibility issue.  You would need to use Chrome or Firefox then download the pdf again.  You should then be able to open it.