If you are entering a W-2, or a 1099, the state abbreviation and state tax ID number are in box 15. If you live in a state that doesn't have a state income tax, this may be blank on your copy of the form, and it's fine to leave it blank in the TurboTax program when you don't have any state tax withheld.
If box 15 is blank, but you need to enter a state ID number, you can use the Federal ID number, which is usually under the employer's name and address.