Schedule 3, Line 12d in TurboTax Deluxe

You can find this by following these steps:

1. Click on Federal Taxes and then Deductions & Credits.
2. Locate the section Other Deductions and Credits 
3. Click Start (or Revisit) beside Other Deductible Expenses.
4. A number of questions will appear on the upcoming screens. You are looking for the one that reads Claim of right repayment over $3,000. Click yes when you see this screen. (Be sure to ignore the similar question, repayments of $3,000 or less.)
5. Enter the amount of the repayment in the box Claim of right repayment over $3,000 and click Continue.

Thanks JeffreyR77.  I see how to do this as an "Other Deductible Expense", but it is then entered as a deduction for 2020 (below Federal standard deduction), and not a credit. As I understand Publication 525, I am allowed to calculate the tax on repayment amount take as credit on Schedule 3, Line 12d (note IRC 1341 in entry line).  However, I have not found the way to do this in TurboTax Deluxe.  I can view the Schedule 3 form under "Forms", but seems I cannot make changes.

Aha, found it.  I needed to open the Schedule 3 under Forms, and then double-click into the Line 12d field, which opens a worksheet to enter the relevant amount and info for Claim of Right IRC 1341 credit for repayments.

I found this discussion while searching for a way to manually input a number into Schedule 3 12d "Other" and it is not for a IRC 1341 credit or credit for U.S. tax paid to Virgin Islands. Is there a way to do this?

Yes.  

You can find this by following these steps:

1. Click on Federal Taxes and then Deductions & Credits.
2. Locate the section Other Deductions and Credits 
3. Click Start (or Revisit) beside Other Deductible Expenses.
4. A number of questions will appear on the upcoming screens.
5. Choose what fits or enter as other

In reply to JeffreyR77

The instructions above, in TurboTax Online ("TTO") work for Method 1 of the IRS 

** form lines based on Tax Year 2021 ("TY21") **

Use TurboTax Desktop ("TTD") for Method 2 

Open Forms mode and open Schedule 3

• click on line 13d and click on the magnifying glass
• this brings you to the 1040/1040SR Wks
• click on line E in Other Credits and Payments Smart Worksheet
  • enter the calculated tax related to prior years by clicking on the + sign
  • enter each tax year separately - if your taxable income is zero, it would generally be the tax line below taxable income 100% (in tax history 5 year worksheet if you have history)
  • this will populate line 13d in the 1040/1040SR Wks ("worksheet")
  • this will also populate Schedule 3, line 13d
  • this will also populate Form 1040 line 31 (Tax Year 2021) to provide the CREDIT (method 2)