You will first have to wait to see if your tax return is accepted or rejected. That probably won't happen until after February 12, when the IRS is scheduled to start processing 2020 tax returns. If it's rejected you can add the alimony when you correct the reason for the rejection, and resubmit the tax return.

If your tax return is accepted, you will then have to wait until the IRS finishes processing it and you get your refund. Then you will have to file an amended return, Form 1040-X, to add the alimony. Here is a link to the instructions for filing a 2020 amended return in TurboTax Online.

How do I amend my 2020 TurboTax Online return?

Thank you for your response. So I cannot amend until after I receive my refund? I'd like to amend immediately since it will change my refund amount.  I guess I will cross my fingers and hope for rejection! The Turbo Tax instructions do not mention having to wait until after I receive my refund.

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@cruxxie wrote:

I'd like to amend immediately since it will change my refund amount.

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@cruxxie  Filing an amended return right away will not stop or modify the processing of your original tax return. The original return and the amended return are processed separately. You will still get the refund shown on the original return, even if you have filed an amended return. Since you will be reporting additional income on the amended return, you will have to pay back part of the original refund. There is no way to avoid this.

There is always a possibility that the IRS will change something on your original return, unrelated to the omitted alimony income. If that happens, you have to take the IRS change into account in your amended return. That's why you have to wait. If they make a change to your original return, and you have filed an amended return that does not take the change into account, it will cause confusion, a lot of correspondence between you and the IRS, and resulting delays.