Box 6 needs an entry if there is an entry in Box 5 or 7. If you only have one state tax return, it isn't necessary to indicate that the income belongs to that state. It will carry forward to your state return.
If boxes 5 and 7 show $0 then make sure you left them empty and didn't manually enter 0 on the form. Same thing happened to me and doing this let me e-file without needing to fill box 6.