The IRS states -
If the taxpayer is amending their 2020 calendar year return, they should manually enter “2020” in the entry box next to “calendar year” on the line below the calendar year checkboxes
You will not be able to e-file the amended tax return if you write in "2020" on the Form 1040-X. It will have to be printed, signed, dated and mailed by you to the IRS.
Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.
See this TurboTax support FAQ on amending a tax return originally completed and filed using the 2020 TurboTax online editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577
The IRS Form 1040-X for amending a 2020 tax return is scheduled to be available on 03/25/2021 (subject to change)
