Since tax year 2020, 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).
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.
If you bought the wrong version, you can get a refund and then buy the one you need, by using this form to get a refund. Make sure to add a clear photo of your receipt, showing the name of the store, the date, the product bought and the price paid.
Thank you for choosing TurboTax.