I cashed out my 401K and paid the taxes at that time. I forgot to put it on my 2021 tax return. Should I amend it?

[DoninGA]

Yes, you will have to amend the 2021 tax return you filed to add the Form 1099-R you received for the distribution.

An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 16 weeks or longer to process an amended tax return.

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 for amending a 2021 return - https://ttlc.intuit.com/community/amending/help/can-i-e-file-my-1040x-to-amend-my-return/00/26008

[VolvoGirl]

Definitely amend.   The withholding taken out might not cover the tax due on your total income.  And you might owe the 10% Early Withdrawal Penalty if you were under 59 1/2.

You should not amend until the first return has fully processed and you get the first refund or your tax due payment has cleared.  

How to Amend

https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...