The method setSheetIndex() does not exist on PhpOffice\PhpSpreadsheet\Writer\IWriter. It seems like you code against a sub-type of PhpOffice\PhpSpreadsheet\Writer\IWriter such as PhpOffice\PhpSpreadsheet\Writer\Html or PhpOffice\PhpSpreadsheet\Writer\Csv.
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation