UnsupportedOperationException in java

I have a problem with this, i dont know why it has a runtime error like this

"Exception in thread "main" java.lang.UnsupportedOperationException". 

It happen when I try to use method remove and addAll of ranked. Help, please.

public class climb {
     public static int climbingLeaderboard(List<Integer> ranked, List<Integer> player) {
            // Write your code here
            List<Integer> Num= new LinkedList<>();
            boolean flag= false;
            
            ranked.addAll(player);
            Collections.sort(ranked);
            for(int i=0;i<ranked.size()-1;i++){
                if(ranked.get(i)== ranked.get(i+1)) {
                   ranked.remove(i+1);
                   i--;
                }
            }
            
            for(int i=0; i<player.size();i++){
                int k = 0;
                for(int j=0; j<ranked.size();j++)
                    if(player.get(i)== ranked.get(j)) {
                        flag= true;
                        k=j;
                    }
                if(flag) {
                    Num.set(i,k);
                    flag= false;
                }
            }
            return ranked.size();
            }
            public static void main(String args[] ) {
                List<Integer> ranked = new LinkedList<>();
                ranked= Arrays.asList(100 ,90, 90, 70, 50, 50,50,40);
                List<Integer> player = new LinkedList<>();
                player= Arrays.asList(95,102,70,60,30);
                System.out.println(climbingLeaderboard(ranked, player));
            }