It's hard to say because you didn't show us what it looks like. (We can't see your screen.) So I'm just guessing. Make sure there's nothing in the box that you're not seeing. Move the cursor all the way to the left and the right to make sure you see everything. Try removing spaces and hyphens, and just string the numbers together.
How many digits is the employer's state ID number on your W-2?
What state did you work in?
What is "it" that says the number is too long? TurboTax? Do you get that message on the screen where you enter the W-2, or in Smart Check when it checks your return for errors? What is the exact error message, word for word?