If it's not allowing you to skip that section and leave it blank, just put in any combination of numbers in this specific format 5 digits, space, 5 digits (for example, 12345 67890). Any numbers will work.
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.