I assume you mean line 18d on Form 1040 for 2019, which is the total payments and refundable credits from Part II of Schedule 3.
TurboTax Free Edition does not support anything for 2019 tax returns. All editions of TurboTax Online, including Free Edition, are only for the current tax year, which is now 2020. If you need to file a 2019 tax return you have to purchase the CD/Download TurboTax software for 2019. There is no free filing for past years. Any edition of the 2019 CD/Download software will support Schedule 3 and Form 1040 line 18d.
Last year, when TurboTax Free Edition was producing 2019 tax returns, it did not support Schedules 1, 2, or 3, so it did not support line 18d on Form 1040.
The 1040 Schedules (1, 2, & 3) are not supported by TurboTax Basic (Free). Moreover, you can no longer file a 2019 return online, so the Free version is not available. To file a prior year tax return, see:
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