I've entered my employers state ID number (KY) from box 15 on my W2 and it's saying that my ID number is incorrect. However, it was the same as last year's, which worked.

Im having a similar problem but my employer's id is nine numbers instead of six any suggestions??

Mine is 6 digits, the same as last year. It tells you to put 3 zeros in front of 6 digit number. It’s rejecting state form says Schedule Ky -2  employer state ID number wrong   Please help me.  
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Have you received a rejection code?  What is the rejection code?  Please clarify.

IRS Instructions for Forms W-2 and W-3 page 24 states: 

Boxes 15 through 20

Use these boxes to report state and local income tax information. Enter the two-letter abbreviation for the name of the state. The employer's state ID numbers are assigned by the individual states.

This TurboTax Best Answer states:

However, with respect to the Box 15 state number ID issue, it won't really matter what you input in Box 15 Employer ID, as long as the software will accept it (explained below).  Make sure that the state where you work is correctly selected from the Box 15 drop-down menu, if it has not already been pre-populated by the TurboTax program.

Then, simply try leaving the number space entirely blank at first.  If there is any issue with Box 15 that prevents e-filing your return with a blank Box 15, and it absolutely requires some value in the software, then either try using your Federal EIN again here, 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,  for instance, then enter a just enter a value that conforms to that format . . . such as 123456XYZ.  This will enable you to bypass the issue and file your return.  In other words, try to be a little creative here, and just give the software some value (even gibberish) that it will accept.

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; the state 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, as long as you can satisfy the software with some acceptable input (or none at all).

@wbrummer