My 2020 return has been accepted by the IRS. Why do I need to wait until the end of March to file an amended return?

Did you accidentally press the amend button?

https://ttlc.intuit.com/community/amending/help/how-do-i-cancel-or-undo-an-amended-return-in-turbotax-online/01/26315

you should wait until your return is approved and processed.

Meanwhile TurboTax doesn't turn on Form 1040-X until later.

That's their policy. Later you will be able to e-File 2020 Form 1040-X. This will be much faster than filing on paper.

Okay so, you’re saying “approved” and “processed” are steps after the return is currently “accepted” by the IRS? How could TurboTax know the IRS would have that done by then?

If you get a refund, you can check your refund status online.

If you don't get a refund, you can assume your return was approved until you hear otherwise in a letter from the IRS.

often the IRS corrects errors for you and will notify you and then you don't have to file any amendment.

lol there is no refund. The entire reason I want to amend is because I owe the IRS money. I want to reduce my tax bill. My return is currently “accepted” by the IRS yet I can’t make an amendment until the end of March for some inexplicable reason.