I assume you are in the process of efiling and referring to the 6-digit code to verify. If you close the TurboTax window that asked for the code and leave to get your emailed code, the code will then be seen as invalid or expired.
You have to keep the window open that asks for the code. You must use either a second window or a second tab to check your email. All common browsers can open a second window or a second tab. Another option is to check your email in a different browser while keeping the first one open, but you shouldn't have to do that.
Here's another option if you have a smart phone. On your computer you can leave the TurboTax window open while you then use your phone's browser to check your email.
