I looked at the diagnostic file that you generated and discovered where the two amounts in question are coming from.
The state tax addback amount of $1976 that appears on line a of SC1040 comes from the State Tax Add worksheet in the South Carolina Individual section when you're in Forms mode in TurboTax Desktop. This is the amount by which your itemized deductions on your Federal return exceed your standard deduction.
The line 2e amount of $1978 for the State and local tax adjustment is because of South Carolina's non-conformity to the provisions of the One Big Beautiful Bill for state and local tax deductions. On your schedule A, your state and local tax deduction total is $13,954. Your SC return already added back $1976 from the amount that exceeded your standard deduction, so that leaves $11,978 (13954-1976). Since the limit for SC state and local tax deduction goes by the prior rules, the maximum state and local tax deduction is $10,000. This means that $1978 ($11,978-$10,000) is entered back due to SC non-conformance to the One Big Beautiful Bill - this is the amount on line 3 of the OBBB Smart Worksheet for line 2e. Because of this, the numbers you are seeing are correct.
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