I received a letter from the IRS stating that I have a tax credit from 2019. Where do I enter that on my tax form?

If the notice you received says you have a credit on your account and there is no mention of it being sent to you, then it will normally be refunded to you automatically with your refund when you file your current year tax return, or by a separate check. It would likely not be appropriate to enter it on your current year tax return, as the only payments you are to enter are estimated payments designated for the current year and a prior year refund that you chose to apply to the current year. 

When the IRS says you have a credit, if usually means you made a payment that has not been applied to a tax return yet. Usually they will give you an option to determine whether that credit will be sent to you in the form of a check, or applied to next year's taxes, or sometimes they will ask you why you made the payment so it can be applied properly.

If the IRS said the payment will be applied to your 2021 tax return, you can enter it as an estimated tax payment. You enter estimated tax payments in the Deductions and Credits section of TurboTax, and then Estimates and Other Taxes Paid.