The Form 8812 computation by the TurboTax program appears to be correct.
Where you’re going wrong is on Line 9 of the Line 5 worksheet in 2021 Instructions for Schedule 8812.
Line 9 says, “Subtract line 8 [of the Line 5 worksheet] from Schedule 8812, line 3.
If line 8 is $150,000, and line 3 of Schedule 8812 is $126,538, then line 9 is a negative number, and line 9 says that if zero or less, enter zero.
A zero on line 9 of the worksheet means that line 10 of the worksheet – “Multiply line 9 by 5% (0.05)” – is zero.
That makes line 11 of the worksheet – “Enter the smaller of line 7 or line 10” – zero, and line 12, which is carried over to line 5 of Schedule 8812, $13,200.
