I estimated and paid capital gains taxes 2 times in 2021. How do I enter the amount I already paid into TurboTax as part of my 2021 tax preparation?

Your payments are estimated tax payments which will serve as a credit against your Federal tax liability and have to be entered on your tax return.

You can enter estimated taxes you paid by following these steps:

1. In TurboTax, open your tax return
2. Click on Federal in the left-hand column, then Deductions & Credits on top of the screen
3. Scroll down to locate the section named Estimates and Other Taxes Paid and click Show more
4. Click on Start next to Estimated Tax Payments
5. On the next page, click Start next to Federal estimated taxes for 2021 (form 1040ES)

Thank you.  This advice was exactly what I needed.

You're welcome!

I have entered the estimated taxes we paid throughout the year but don't see where to enter the taxes we paid with our return

You don't enter a tax due paid.  That is not deductible.  Unless you paid a state tax due.  That can be an Itemized Deduction.

You can deduct any state taxes paid (for any year) or state estimates actually paid in during the year on your federal return if you Itemize deductions.  But just the tax, not any interest or penalties.   If you transferred from last year it might already be filled in.

Enter it under

Federal on left

Deductions and Credits

Then scroll way down to Estimates and Other Taxes Paid

Other Income Taxes - Click the Start or Update button

Next page scroll way down to the last section, Other Income Taxes Paid