I believe it's on the Schedule D screen. Scroll down near the Sale of Home section, and above that there are two sections - one to enter prior year info (from year of sale) and below that you can enter principal payments by year if you want. Hopefully that's what you are looking for.

https://proconnect.intuit.com/support/en-us/help-article/federal-taxes/enter-prior-year-installment-...