If you have already filed your return, you'll need to wait for your tax return to either be rejected or accepted, if it is rejected you can add your Form 1099, correct any other errors on the return and resubmit it.
If it is accepted, you will need to amend your return to include the Form 1099.
Please be aware that Form 1040X will not be available in TurboTax until February 18, 2021, but once it is available, you will be able to prepare and e-file your amended return.
Please see this TurboTax FAQ for instructions on how to amend your tax return in TurboTax.
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