Correct, you have to file an amended 2017 tax return, which I just completed. Unfortunately no ruling (to the best of my knowledge) has been made yet on extending this to 2020...I hope they do as their are several homeowners still unware that they have this issue.
My question was how to get this casualty loss entered into TurboTax. You can take the safe harbor procedures but I didn't see this as an option in Turbo Tax. I ended up entering the amount I paid out of pocket as the fair market value before the casualty and put $0 as the fair market value after the casualty and ended up crossing these off the form 4684 as it is my understanding the safe harbor amount should be listed directly on line 7 and put "Revenue Procedure 2017-60" at the top of the form, which I entered manually.
