missing letter(s) is easily fixed
    if (arr.length==letters.length) {
      var s='[';
      var t=',';
      for (var j=0;j<arr.length;j++) {
        if (j==(arr.length-1)) t='],<br>';
        s=s+"'"+arr[j]+"'"+t;
      }
      document.write(s);
    }
so too is 'no country is the same as the answer'
        if (l==first_letter && c!=c0) {
but 'repetition' is not so easy

We need to ask is a country already in the array we are building (arr)?

This means we need a way to ask the question, "Is this item in this array?"

JavaScript offers the indexOf function, returning -1 if the item is not in the array.

        if (l==first_letter && c!=c0) {
          if (arr.indexOf(c)==-1) {
            arr.push(c);
            break;
          }
        }