Most times if your W-2 doesn't have anything in Box D, you can just skip it. But if you are getting an error when trying to skip in TurboTax, you can just put in any combination of numbers in this specific format 5 digits, space, 5 digits (for example, 12345 67890).
That is just a number that the payroll team uses to identify your W-2 for their system purposes.This is not something the IRS needs, so as long as you input something in this format, you can proceed.
If you are trying to import, click Skip import and key it in manually. This workaround won't work when importing.
You can find more details on Box D and the Control Number here https://ttlc.intuit.com/questions/2669624