the carryback is an election you can make by checking the box on form 6781
it is an election, not a requirement. first net 1256 current year losses must be used to offset any current year capital gains (or it adds to other current year capital losses). if you still end up with a net capital loss, it becomes a carryforward unless there is computed a remaining 1256 loss which you can elect to carryback. 2020 would go back to 2017 but read the rules on the form for the amount that can be used in 2017. any remaining loss goes to 2018 then 2019 and if there's still a loss remaining it goes forward to 2021
here's the link to the federal form which also has the instructions. the one about the carryback starts on page 2.
notice the IRS terminology "can elect"
https://www.irs.gov/pub/irs-pdf/f6781.pdf
this is from IRC code section 1212
(c)Carryback of losses from section 1256 contracts to offset prior gains from such contracts
(1)In general
If a taxpayer (other than a corporation) has a net section 1256 contracts loss for the taxable year and elects to have this subsection apply to such taxable year, the amount of such net section 1256 contracts loss—
I'm saying your tax firm is wrong if that's what they told you.
