#include<iostream>
//#include<math.h>
//#include<stdlib.h>
using namespace std;
int main()
{
    int score,n;
    cin>>score;
    //if(score>0)
    //{
        n=score/10;
        switch(n)
            case 10:
            case 9: cout<<'A';break;
            case 8: cout<<'B';break;
            case 7: cout<<'C';break;
            case 6: cout<<'D';break;
            default: cout<<'E';
    //}
    return 0;
}

switch(表达式)
{你的这个大括号没有
case 常量表达式1:语句1;
....
case 常量表达式2:语句2;
default:语句;
}

还应注意单双引号的区别

#include <iostream>
using namespace std;

int main()
{
    int score ,n;
    cin>>score;
    n = score/10;
    switch(n)
    {
    //case 10:
    //    break;
    case 9:
        cout<<'A';
        break;
    case 8:
        cout<<'B';
        break;
    case 7:
        cout<<'C';
        break;
    case 6:
        cout<<'D';
        break;
    default:
        break;
    }
    return 0;
}