int main()
{
    int n , m;
    cin >> n >> m;

    int *counter = new int [n];

    for ( int i = 0 ; i < m ; i++ )
    {
        int temp;
        cin >> temp;
        counter[temp-1]++;
    }

    for ( int i = 0 ; i < n ; i++ )
    {
        float result = (float)counter[i] * (float)100 / (float)m;
        printf ("%.2f%%\n" , result);
    }

    delete[] counter;

    return 0;
}

(WA On Test 1)

Where is "cout"?

printf() also does the same work as cout<<

I think the problem is from using int*counter.. Try using vector<int>counter(n); instead..
It worked fine when i tested it with the rest of your code