The Control number on your W2 is an internal number used by your payroll administrator. It is not a number that is used by your State, or my IRS.
If you do not have a control number, you can leave that field blank, or add in any 10 numbers with a space after the first 5. For example (12345 67890)
If you are getting an error message when you are trying to upload your W2, you will need to manually enter your W2.
Here is a Help article with some more details:
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