In the case of when you didn't actually file your 2016 yet, you should be able to enter "0" as the AGI for the e-file verification and that should allow you to e-file.
If that doesn't work - sometimes the IRS verification for this is a bit funky and, at this point in season, since you have to mail 2016 anyways, it might be easiest to also mail 2017. But, in separate mailings.
