Inicialize o valor antes de usá-lo:
if (!isset($assignedQuantityPerUser[$row["sellerid"]])) {
$assignedQuantityPerUser[$row["sellerid"]] = 0;
}
$assignedQuantityPerUser[$row["sellerid"]] += $totalUnitsOrdered;
if (!isset($assignedQuantityPerUser[$row["sellerid"]])) {
$assignedQuantityPerUser[$row["sellerid"]] = 0;
}
$assignedQuantityPerUser[$row["sellerid"]] += $totalUnitsOrdered;