Can I go back and enter the 2019 federal taxes I paid?

Have you already filed?  What taxes did you pay?  How?  Did you send in estimated taxes?

If  you haven't filed yet,

You can type estimates paid in the search box at the top of your return and click Find and it will give you a link to Jump To it.

OR
To enter Federal or State Estimated Taxes Paid, including a state estimated payment made in Jan 2019 for 2018 (or Jan 2020 for 2019), go to
Federal or Personal (for Home & Business version)
Deductions and Credits
Then scroll way down to Estimates and Other Taxes Paid
Estimated Taxes - click the Start or Update button

For 2019 Estimated payments go on 1040 Schedule 3 line 8
And show up on 1040 line 18d