If the loss occurred in 2020, you claim the loss by filing an amended 2020 tax return.
But here's the problem. The Federal Disaster Tax Relief Act of 2023 (which was signed December 2024, despite the name) changed the rules on deducting losses, so that a loss you did not deduct in 2020 because of the old rules, might be deductible now. However, the Act did not explicitly extend the deadline for filing amended returns. The deadline for filing an amended 2023 return was May 15, 2024 (or, two years after the date you did file if you filed late). So technically it is too late.
However, it is possible that the IRS will waive the deadline because the law is a bit confusing on this point. To claim a loss that occurred in 2020, you can file an amended 2020 return, and see what the IRS says.
https://ttlc.intuit.com/turbotax-support/en-us/help-article/amend-tax-return/federal-disaster-tax-re...
https://www.westerncpe.com/taxbyte/federal-disaster-relief-act-makes-big-changes-to-casualty-loss-de...
Note that you must include the FEMA number of the disaster on your tax return, so don't even try to file for Hurricane Laura on the wrong year tax return, that will get denied.
