the way I read that code section is because 2017 is filed more than 3 years after its due date including any valid extensions (if the taxpayer took out a valid extension for 2017, they have until 10/15/2021 to file it and would be able to file NOL c/b claims) the NOL can't be carried back. since it would seem no valid carryforward election was made there is no carryforward either. There is nothing that prevents the taxpayer from filing 2017 and then carrying back the NOL. the worse that can happen is the IRS rejects the c/b claims. However, say the IRS honors the C/B claims but then realizes that the law doesn't allow it. the taxpayer will get a bill.
