The Employer I.D. No. is missing from Box 15. What do I do?

That shouldn't actually be a problem.  The only truly meaningful elements on a W-2 for income tax filing purposes are your employer's federal EIN (Employer's tax Identification Number), your own Social Security Number (SSN), and your actual wage data.  If there was a mistake made with any of these items, then that is a cause for concern.  A missing State Employer ID Number in Box 15 of a W-2, on the other hand, is not a big issue.

In fact, the IRS computers and your state department of revenue really won't care, as they're really just looking for the federal EIN, SSN, and relevant wage data.  The State ID is simply a bureaucratic convention for your state; they can and will still be able to easily and uniquely identify your employer by their federal EIN, even if they have to cross-reference the Federal / State ID numbers in their database.  In other words, this single issue should not affect the processing of your tax return.

Therefore, you can try to just re-enter your Federal ID Number into Box 15 if you like, or you can leave it blank.  Alternatively, you can go to your employer's payroll or human resources office and ask if they have applied for a company state ID number, or if one exists for your W-2.

If there is any issue with Box 15 that prevents e-filing your return, and it absolutely requires a value in the software, then either use your Federal EIN, or enter a "nonsense" value such as 12345 67890 (with a space in the middle, or not).  If the TurboTax program asks you for a state ID number in a specific format, such as needing exactly 9-digits and ending in XYZ, then enter a value that conforms to that format . . . such as 123456XYZ.  This will enable you to bypass the issue.

Again, it is not a material factor to the filing of your income tax return, and it will not change the value of your refund (or liability) by a single dollar.

Thanks for asking this question, and good luck to you!