How to correct my prior year (2018) state tax payment for schedule A while calculating the itemized amount. I didn't filed 2018 state tax through TurboTax.

Please view this IRS link to access Form 1040-X and read the instructions to prepare and mail your 2018 amended return. You will need your 2018 original return when preparing the amendment. If you do not have a copy of the return, then you can request an IRS transcript.