Try something like this

const sumAll = function(num1,num2) {
    if (num1 < 0 || num2 < 0) {
        throw "Num1 and num2 must not be negative"
    }
    let newArr = [num1,num2];
    let sortedArr = newArr.sort(function(a,b) {
        return a - b;
    })
    let lastArr = []
    for (let i = sortedArr[0]; i <= sortedArr[1]; ++i ) {
        lastArr.push(i)
    }
    let reducedArr = lastArr.reduce(function(previousValue,currentValue) {
        return previousValue + currentValue;
    })
    return reducedArr;
};

console.log(sumAll(1,-4));

If you want to return an error message instead of throwing an error, replace the word throw with the word return on line 3.

If question is related to negative parameter check, I think you can use Math.sign() function for checking negativity/positivity of values.

Math.sign(-4) returns -1

Math.sign(4) returns 1

In if case it will be represents true/false

const sumAll = function(num1,num2) {
  if (Math.sign(num1) || Math.sign(num2)) {
    return "Parameters must be positive integer."
  }

  let newArr = [num1,num2];
  let sortedArr = newArr.sort(function(a,b) {
    return a - b;
  });

  let lastArr = [];

  for (let i = sortedArr[0]; i <= sortedArr[1]; ++i ) {
    lastArr.push(i);
  }

  let reducedArr = lastArr.reduce(function(previousValue,currentValue) {
    return previousValue + currentValue;
  });

  return reducedArr;
 };
    
console.log(sumAll(1,-4));