Not sure what you really mean by "social security benefit sstatement was not attached" since it would have been impossible to "attach" anything to an e-filed tax return. So....do you mean that you did not enter your SSA1099 when you prepared the return, and now you need to amend the return?
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
Since you will need to mail the amended return, you can attach a copy of the SSA1099 with the 1040X that you mail in.
Go to Federal> Wages & Income>>Retirement Plans and Social Security (SSA1099 and 1099RRB) to enter your SSA1099.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
