Here is from Form 6781 instruction:
The amount you can carry back to any prior year is limited to the smaller of:
• The gain, if any, that you would report on line 16 of Schedule D (Form 1040) for that carryback year if only gains and losses from section 1256 contracts were taken into account; or
• The gain, if any, reported on line 16 of Schedule D (Form 1040) for that carryback year.
So I suggest you to check your 2021 return, use the GAIN number on Sched D, line 16 as your carryback amount.
