You need to install TurboTax (or associate the file with TurboTax) as that is the problem. As Svante has said, it has nothing to do with AxCrypt. You’ve managed to decrypt the file with AxCrypt. That means AxCrypt has done it’s job properly (encrypt -> decrypt)
Once you’ve installed TurboTax your files should open. If you’re still having problems, right click on the .tax2016, find Open With, select TurboTax.
You may want to take your computer to a local repair shop and pay them to assist you. This is not a problem with your computer, more of a lack of your understanding how to use a computer. There are plenty of free, online computer courses and you’ll be less frustrated if you understand why these problems are arising.