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according to https://barcode-coder.com/en/ean-13-specification-102.html the checksum is not calculated as stated in the puzzle.
The problem statement does not explicitely tell you how the checksum was calculated, but instead gives you the way to verify that the barcode is consistent with its checksum.
You can verify that both operations are perfectly consistent with each other: Your link says
checksum = -(3*Odd + Even) mod 10 while the statement says
3*Odd + (Even+checksum) = 0 mod 10.
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