You can file an amended return within 3 years after the date you filed your original return or within 2 years after the date you paid the tax, whichever is later.
The six month limit for choosing which year to take the casualty loss applies only to this election of which year (2015 or 2016, in your case), not to the ability to take the loss at all.
You are still able to amend your 2016 tax return to claim the casualty loss.
