I forgot to add state taxes i paid in 2018

To enter estimated taxes paid (Federal, State, Local)

•    Click on Federal Taxes (Personal using Home and Business)
•    Click on Deductions and Credits
•    Click on I'll choose what I work on (if shown)
•    Scroll down to Estimates and Other Taxes Paid
•    On Estimates, click on the start or update button

If you already filed the State should catch this and give you credit to the state taxes paid.  If not then you will need to Amend the state return.