Update OS needed for TurboTax

Other possibilities.

How to get macOS High Sierra   

Sierra download

Check to make sure your applications are compatible.      

 Application Compatibility

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

What model computer do you have?

Go to the apple in the upper left - select about this mac

The replies about updating assume you can do that and many Apple computers do not meet upgrade requirements.

If you are using 10.11.6 because that is as high as you can go, you will need to:

1) Find an old copy of software somewhere;

but since tax software is usually specific for each year

old copies will likely not help with this years taxes.

2) Contact TurboTax for a solution.

3) Use a different tax software.

4) Or hire someone or do them the old by hand way.

See the following:

macOS - How to Upgrade - Apple