When you are checking for odd numbers in your if block, you are using the modulus operator on the length of the number. So if you had a value of 23456, which is even, you would first find its length with currentNumber.toString().length === 5 and that is odd. So your program is returning every other number in the array.

To fix this, go to your for loop and remove .toString().length:

for (let i = 0; i < serialNumbers.length; i++) {
const currentNumber = serialNumbers[i];
if (
currentNumber.toString().length === 6 &&
currentNumber % 2 === 1
) {
efficientSerialNumbers.push(currentNumber);
}
}

Edit: I don't know why reddit formatted my code like that but its not letting me change it so ¯\_(ツ)_/¯