You can't choose when to use a capital loss carryover. It has to be carried over from year to year until it's used up. You can't skip years, and you can't just take the carryover from your 2017 tax return and enter it in 2020. Some of it would probably be used each year, even if you did not have any capital gains. If you had a capital loss in 2017 and you could not deduct the entire loss that year, you have to carry it over to 2018, then to 2019, and then to 2020 if it is not all used up earlier. You will have to amend your 2018 tax return to include the capital loss carryover. If it's not used up in 2018, you will have to amend your 2019 tax return as well. Then you will be able to enter the information from your amended 2019 tax return in TurboTax for 2020, so it can calculate the amount that is carried over to 2020, if any.
For all years, when TurboTax asks if you know the carryover amounts, do not answer Yes. Select the option that says you need help figuring out the carryover amounts.
You don't have to wait for your past-year amended returns to be processed before you file your 2020 tax return, but you need the information from the recalculated 2019 tax return in order to correctly calculate the capital loss carryover to 2020.
