I entered my Roth IRA contribution for 2019 into TurboTax, but it doesn't appear on my return. It does for 2017 and 2018. Does anyone know why?

ROTH Contributions don't show on your tax return.  They are not deductible.  Where did they show last year?

Enter IRA contributions here:
Federal Taxes,
Deductions & Credits,
I’ll choose what I work on (if that screen comes up),
Retirement & Investments,
Traditional & Roth IRA contribution.

OR Use the "Tools" menu (if online version under My Account) and then "Search Topics" for "ira contributions" which will take you to the same place.

A Roth IRA contribution does not actually go on a tax return, but you should enter it anyway to:

1) Tell you if your income qualifies you for a contribution and warn you if it does not.
2) Check if your income exceeds the limit to contribute to a Roth.
3) Track your contribution year-to-year if you use TurboTax every year.
4) Add the Retirement Savers Credit if you qualify.