Under Deductions > Adjustments to Income, on the Retirement tab, under the Traditional IRA section, on the line for 2019 payments from 1/1/20 to 4/15/20.  That's not the most appropriate description for that line but that's the line you'd use.

I filled out that line and it's still not populating. Any idea why?

That's because you didn't populate Line 1 for nondeductible contributions.