If you are referring to the Control number Box D on your W-2, you can either skip that section and leave it blank. Or if you are receiving an error when trying to leave it blank, 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.
NOTE: If you are trying to import, click Skip import and key it in manually. This workaround won't work when importing.