I understand now. The error message is misleading in two ways.
1) The reference to Line 15G is referring to the TT Smart Worksheet embedded in TT's version of Form 8379. The actual IRS Form 8379 doesn't have a Line 15G or any Line D-G. But the actual IRS form does have Lines 15 A-C, which is what the back half of the error message is referring to, not Lines A-C in the Smart Worksheet. The error message might be more constructively written as "for each Line 15 A-F, in the Smart Worksheet, the Joint column must be equal to the sum of the Injured Spouse and Other Spouse columns".
2) The form highlights the cell in Line 15G(Joint) as being the problem, which is an auto-calculated (correctly) value/sum of the values Lines A-F immediately above it. The values immediately above it are auto-filled by TT on behalf of the user. If the individual Smart Worksheet cell(s) in question (incorrect sum) in the Joint column were highlighted, that would have been a more appropriate UI design choice.
I manually allocated Line 15B within the Smart Worksheet and addressed the error.
