I fixed it for any array length, and it kept failing the test cases. I then had it print out the results, and it was getting the correct totals, but then ending with an unspecified error. I just hit submit final and it got accepted.

function squareSum(numbers){
var numSum = 0;
for (var i=0;i<numbers.length;i++){
numSum+=(Math.pow(numbers[i],2));
}
return numSum;
}

There should be array with different lenght, not just 3 ;)

reduce doesn't do exactly what you think ;)