How do I enter social security benefits on SSA-1099 for a dependent (parent) - the TurboTax form only lists myself or my spouse for data entry?

You do not enter a dependent's income on your own tax return.  If your parent only has income from Social Security then it does not have to be filed on a tax return.

Thanks - years past we've had it listed in our return - so as I understand it doesn't need to be filed at all?

No, if your parent's only income is SS, then they do not need to file a return.  And it should never have been entered on your return.  That made it look like your income was higher than it really was, and may have caused you to pay too much tax.  You can amend as far back as 2015--but amend 2015 by April 15, 2019 or it will be too late.