You just need to change the words[i] === '' into words[i] === ' '. '' is an empty string.

Another option is splitting the passed string.

function makeAbbr(str) {
   // words is [ "central", "processing", "unit" ]
   let words = str.split(/\s+/);
   let abbreviation = '';
   for (let i = 0; i < words.length; i++) {
     abbreviation += words[i][0];
   }
   return abbreviation.toUpperCase();
}

This will generally work on abbreviation for words that use the first character of each words separated by a space.

function makeAbbr(words) {
  // result abbreviation
  let abbreviation = '';
  // store each word into an array using split by space
  let wordArray = words.split(' ');
  // iterate through the word array
  for (let i = 0; i < wordArray.length; i++) {
    // take the first character in each word into the result abbreviation
    abbreviation += wordArray[i][0];
  }
  // return the abbreviation with all of them being upper case
  return abbreviation.toUpperCase();
}
// test case
console.log(makeAbbr('central processing unit'));

A one liner solution:

function makeAbbr(words) {
  return words.split(' ').map(word => word[0].toUpperCase()).join("");
}

console.log(makeAbbr('central processing unit'));

We are converting the string sentence to an array of words separated by a space (.split(" ")), then mapping or transforming every word in the array to its first letter, as well as capitalizing it: .map(word => word[0].toUpperCase()), then joining the array elements into a string with a "nothing" "" as a separator:

"central processing unit" -> ["central", "processing", "unit"] -> ["C", "P", "U"] -> "CPU"