if you actually made an attempt in 2018 to get back some or all of the franchise fees, then 2018 would probably be the right year. This is because you would not know how much to write off until the attempt succeeded or failed. on the other hand if you were only thinking about it, I would amend 2017 because you say that you actually closed your business that year.
here are some possibilities.
say you go back and write it off in 2017, the IRS audits and says 2018. - at least you get the writeoff
say you write off in 2018, the IRS audits, but after 4/15/2021 (10/15/21 is 17 return extended), and says it should have been in 2017. 2017 is a closed year. so you lose the deduction completely
