You can get a replacement here:
https://www.ssa.gov/myaccount/replacement-SSA-1099.html
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If you can't find it in your home SS or tax folders, you may need to file an extension since the deadline for filing is just a week away now.
They certainly mailed you an SSA-1099 if you received an SS check in December 2020.
If you moved right after retiring, they may have sent it to your former address...so make sure they have the correct address.
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The form is an SSA-1099.....it is entered on the Wages&Income page in the tax software ( or the Personal taxes section if you are using Home&Business software).
There is a specific menu item for entry of SSA-1099 forms....do not try to put it in with the standard 1099-R forms.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*