With the new tax law, State and Local Taxes are limited to $10,000 deduction.
I am assuming you already have that amount with the State income taxes withheld from the W-2's, vehicle registration fees, and other local taxes already.
If this is not the reason, then you cannot exceed the new higher standard deduction amount which is $24,000 for a married couple under age 66.
Please see below and click on the SALT deduction and the standard deduction to see these changes that are impacting your return for 2018.
https://ttlc.intuit.com/replies/6971774
