If you have already filed your 2023 income tax return and you have now been notified of a Form 8949 or IRS form 1099-B which was not reported, you will want to report this new income.
The IRS will eventually match up this information with your 2023 income tax return and realize that this information was not reported. You will want to report this information now on an amended tax return as favorably you can.
Is it possible to determine a cost basis which would reduce or eliminate a gain?
If your return has been transmitted and rejected by the IRS, you can correct your tax return and re-transmit.
If your return has been transmitted, accepted by the IRS and your refund issued, you will have to amend your Federal 1040 tax return.
If you used TurboTax Online, log in to your account and select Amend a return that was filed and accepted.
If you used the CD/download product, sign back into your return and select Amend a filed return.
See also this TurboTax Help.