Box 14 is the "State distribution". When there is only one state listed in Box 13, the assumption is that the amount in Box 14 is the same as Box 1 and Box 14 can be left blank (per IRS instructions, it is not required).
Box 14, however, is required by one or more states, and causes an efile error for those states if left blank.
The solution is to enter the Box 1 amount (or Box 2a amount if the taxable distribution is different than Box 1) in Box 14.