As long as you are not trying to import your W-2, you don't have to have a control number, you can just leave it blank.  If you are trying to import and are having trouble you can just manually enter your W-2.

https://ttlc.intuit.com/replies/4417180

If you would like to enter your W-2  manually, here are the instructions for TurboTax Online:

1.      Open (continue) your return if you don't already have it open and click the button Take me to my return

2.      Select Federal in the left navigation panel.

3.      Next select Income & Expenses at the top.

4.      Scroll down to Wages and Salaries and select Show more.

5.      Click Start/Revisit next to Wages and Salaries

6.      Follow the onscreen instruction to enter the W-2.