Why the below code showing error?

function rom(num) {
  if (num >= 1000) {

    const roman1 = convertToRoman(num - 1000);

    roman1.unshift("M");

    return roman1;

  } else if (num >= 900) {

    const roman2 = convertToRoman(num - 900);

    roman2.unshift("CM");

    return roman2

  } else if (num >= 500) {

    const roman3 = convertToRoman(num - 500);

    roman3.unshift("D")

    return roman3

  } else if (num >= 400) {
    const roman4 = convertToRoman(num - 400);

    roman4.unshift("CD")

    return roman4
  } else if (num >= 100) {
    const roman5 = convertToRoman(num - 100);

    roman5.unshift("C")

    return roman5
  } else if (num >= 90) {
    const roman6 = convertToRoman(num - 90);

    roman6.unshift("XC")

    return roman6
  } else if (num >= 50) {
    const roman7 = convertToRoman(num - 50);

    roman7.unshift("L")

    return roman7
  } else if (num >= 40) {
    const roman8 = convertToRoman(num - 40);

    roman8.unshift("XL")

    return roman8
  } else if (num >= 10) {
    const roman9 = convertToRoman(num - 10);

    roman9.unshift("X")

    return roman9
  } else if (num >= 9) {
    const roman10 = convertToRoman(num - 9);

    roman10.unshift("IX")

    return roman10
  } else if (num >= 5) {
    const roman11 = convertToRoman(num - 5);

    roman11.unshift("V")

    return roman11
  } else if (num >= 4) {
    const roman12 = convertToRoman(num - 4);

    roman12.unshift("IV")

    return roman12
  } else if (num >= 1) {
    const roman13 = convertToRoman(num - 1);

    roman13.unshift("I")

    return roman13
  } else {
    return []
  };

}

function convertToRoman(num) {

  let arr = rom(num);

  let atr = arr.join("")

  return atr;
}

console.log(convertToRoman(3));

What is this challenge, what error is it showing?

I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

You can also use the “preformatted text” tool in the editor (</>) to add backticks around text.

See this post to find the backtick on your keyboard.
Note: Backticks (`) are not single quotes (').

The error you see is because you cannot Unshift a value which is not an Array,
return [atr] instead of return atr;

and your problem is solved.