Probably out of luck at this point unless the loss was large.
It must be reported in the tax year that the loss occurred and would offset any capital gains for that year and up to $3,000 of ordinary income. Any loss reaming would carried to the next tax year to be applied the same way. That continues each year to the present if loss remains. No year can be skipped of the loss would be forfeited.
Only tax years 2017-2019 can be amended to claim any remaining carry over. Assuming that that taxpayer had more than $3,000 of taxable income each year since 2011 and no capital earnings to claim, then unless the loss was more than (6 years X 3000=) $18,000 there would be nothing reaming to deduct now. If the loss was more then $18,000 then there is a possibility that after applying the loss to mock amended 2011-2016 tax returns they might be some remaining loss to carry to 2017 (and later years) for a refund.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
