It says Box 18 can't be less than local withholding which is blank. But it's not accepting blank on my Schedule.

Maybe there is a whitespace " " character in box 18? Click in the box, press Ctrl-a (to select all) then press Backspace or Del.

If you've worked in the same place, the box 18 (Local Wages) will be the same as Box 16 (State Wages)