W2 shows state income tax withheld for CT. Employer is not a CT company and there is no Employer state ID number on W2. Why won't turbotax let me leave it blank?

Usually, state tax applies to income earned in that state, regardless of the employer's domicile.

I would fill it in, and apply the net state taxes paid in 2021 toward the (unfortunately lowered) SALT deduction, if it benefits you to take it.

If the space for a state tax ID number is blank on your W-2 form, but state tax was withheld, you can use the EIN  in the state ID field. The EIN is in box b, which is usually near the top of the form. 

@dave171 

Just make sure you check each and every W-2 copy they have on their PDF set of W-2 forms.  Usually, there is more than one W-2 provided in a paper/PDF W-2 set.  One copy for the IRS, and one or several for using with the state filings (In case you end up paper filing by mail)