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.
There is no requirement that your Social Security Benefit Statement (SSA-1099) itself be "attached," either physically or electronically, to your federal tax return, but it does need to be entered into TurboTax so that the amount of the benefits will be present on Form 1040 line 6a and the taxable portion, if any, will be present on line 6b. That doesn't mean that the IRS can't subsequently ask you for a copy, particularly if the amount on line 6a does not match the records provided by the SSA to the IRS.