Often state return reject due to something on the W-2 not matching up. Make sure your rejection error is referring to line 18 instead of box 18. If it says something is wrong with box 18, then double check all the W-2 entries that are part of your return and be sure that if there is anything reported in box 18 of any of your W-2s, that it is a numerical value and that box 19 and 20 are also filled in. If boxes 19 and 20 are blank, but box 18 is zero, then delete the zero.
If your state return is still rejected after double-checking those entries once again, then you should print it and mail the return. Be sure to include copies of all W-2 and other tax forms showing state withholdings.