If this contribution was made through your employer and appears with code W in box 12 of your W-2, on the did your employer tell you about any other contributions page, answer Yes, then enter the amount in the first box labeled Employer and payroll contributions made in 2020 for tax year 2019.  This amount should have been reported on your 2019 Form 8889 in a similar fashion by entering the amount in the second box and that same page in 2019 TurboTax.

If this was instead a personal contribution for 2019 not made through your employer, this should have been entered into 2019 TurboTax and not in 2020 TurboTax.

@dmertz This was a personal contribution (my employer does not contribute to my HSA), and I did include that contribution in my 2019 TurboTax return. My confusion is due to my 2020 5498-SA form, which includes this contribution in box 2's "Total contributions made in 2020". I did not expect that payment to be included here because it was designated for 2019. However, the IRS instructions for the form say it should be included: https://www.irs.gov/pub/irs-prior/i1099sa--2020.pdf. Should I simply subtract that payment from box 2's figure when prompted by TurboTax for my total 2020 contributions?

@dmertz Thank you for your answer...it makes perfect sense. It would really be great if TurboTax included that guidance in their instructions!