A Control Number (box D) is used by many payroll departments to uniquely identify a W-2 in their system. If you get errors when attempting to e-file with an empty box D, just enter any number in this format: 5 digits, space, 5 digits (for example 12345 67890). The IRS doesn't care what's in box D.
This link I have no box D or Control Number on my W-2 has information you may find useful.
@torandafields
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