in 2020 you had a choice if you had an NOL carryover. 1) you could have elected by attaching a statement to the 2020 timely filed return that you wanted to carry forward the NOL to 2021 and future years if need be. 2) if you did not include the election or did not timely file, your only option is to carry back the NOL to the 5th earliest year IE 2015. To the extent not used in 2015 carry the balance forward to 2016 and so on.
The deadline for filing this claim is three years after the filing date (5/17/2021 for 2020 1040) of the return for the tax year in which the NOL arose (or the original due date of that return if it was filed early). it would seem if you did not make the election to carry forward the loss, it is too late to file an amended return for 2015.
