That code means you are entering an invalid license code. Make sure enter exactly the 16-character combination of letters and numbers you received for the license code.
There is a difference between a License Code (for Desktop) and an Activation Code (for Online). This error code 606 means you are trying to enter a Code for TurboTax Online in a Desktop software (or vice-versa).
You will have to contact TurboTax support for assistance with this problem. Here is a link: Turbo Tax Customer Service
We understand sometimes this can be confusing, but on the box (or on the retailer website), you have to choose between TurboTax Online and TurboTax Desktop. They are not the same software, and codes from one cannot be used in the other one.
If on the box it's TurboTax Online, you can activate your code to your account on this link.
If on the box it's TurboTax Desktop, you can download/install your software from this link.