You cannot enter and investment sale, gain or loss, from 2021 on a 2023 tax return. This has to be entered on the 2021 tax return that was filed. You will need to amend the 2021 tax return to report the sale.
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
