It makes no sense to use reduce here, however you could use a new array as an accumulator and do insertion sort with all elements:

array.reduce((sorted, el) => {

  let index = 0;

  while(index < sorted.length && el < sorted[index]) index++;

  sorted.splice(index, 0, el);

  return sorted;

}, []);

Here is the version without reduce:

array.sort((a, b) => a - b);

Now some general tips for writing reducers:

how must be the reduce call back function?

You either take an approach with an accumulator, then the reducer should apply a modification to the accumulator based on the current element and return it:

(acc, el) => acc

Or if accumulator and the elements have the sane type and are logically equal, you dont need to distinguish them:

 (a, b) => a + b

what is the initialValue of reduce function?

You should ask yourself "What should reduce return when it is applied on an empty array?"

Now the most important: When to use reduce? (IMO)

If you want to boil down the values of an array into one single value or object.