If you have not filed your return or if your return has been rejected you can add a state by taking the steps below. 

1)    Sign into TurboTax Online

2)    Continue your return by click Taxes > Tax Timeline > Take me to my return.

3)    Click on the Federal Taxes tab (Personal using Self-Employed w/ Business Tab)

4)    Click on Wages and Income (Personal Income using Self-Employed w/ Business Tab)

5)    Click Jump to full list

6)    On Wages and Salaries, click Start/Revisit

7)    On the W-2 Summary Screen click +Add a W-2.

If you have already filed your return and it has been accepted you will need to amend your tax return. Please see the following FAQ article for instructions. https://ttlc.intuit.com/replies/3288565