How do I submit missed child care expenses from 2017 on my 2018 tax returned?

[DoninGA]

You cannot enter a 2017 expense on a 2018 tax return.  You have to amend the 2017 tax return that you filed to add the expense.

See this TurboTax support FAQ for amending a 2017 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/questions/4252298-how-do-i-amend-a-2017-return-in-turbotax