总的说来，你有主要有两个错误

• 一是录入的时候，分数的录入不对
• 二是冒泡交换代码不对

我按照你代码简单改了下，能运行。
建议你自己一步一步调试，保证每一步正确，多写点代码。

#include <stdio.h>
struct Stpoint {
    int code;
    char name[20];
    int scores[4];
    int sum;
};

int main()
{
    struct Stpoint students[5] = { 0 };
    //加了一个冒泡排序时候的临时变量，另外定义变量最好设置默认值
    struct Stpoint studentstemp;
    int temp;
    int i, j, k, s_sum[5] = { 0 }, temp1, temp2, sum;
    char str[20] = { 0 };
    float ave;
    for (i = 0; i < 5; i++) {
        printf("输入第%d个学生的姓名：", i + 1);
        scanf("%s", &students[i].name);
        printf("学号：");
        scanf("%d", &students[i].code);
        //要想一起输入需要拆分字符串并转换
        printf("科目1成绩：");
        scanf("%d", &students[i].scores[0]);
        printf("科目2成绩：");
        scanf("%d", &students[i].scores[1]);
        printf("科目3成绩：");
        scanf("%d", &students[i].scores[2]);
        printf("科目4成绩：");
        scanf("%d", &students[i].scores[3]);
    }

    for (i = 0; i < 5; i++) {
        students[i].sum = 0;
        for (j = 0; j <= 3; j++) {
            students[i].sum += students[i].scores[j];
        }
        ave = students[i].sum / 4;
        printf("%s  总分为%d,平均分为%.1f\n", students[i].name, students[i].sum, ave);
    }
    /*冒泡法*/
    for (i = 0; i < 4; i++) {                                 

        for (j = 0; j < 4 - i; j++) {
            if (students[j].sum > students[j + 1].sum) {
                /*交换结构数组的成员*/
                //之前代码有错误
                studentstemp = students[j + 1];
                students[j + 1] = students[j];
                students[j] = studentstemp;
            }
        }
    }

    for (i = 0; i <= 4; i++) {
        printf("%d   ", students[i].sum);
    }

    printf("\n");
    for (i = 4; i >= 0; i--) {
        printf("%s   ", students[i].name);
        printf("%d   ", students[i].code);
        for (j = 0; j <= 3; j++) {
            printf("%d   ", students[i].scores[j]);
        }
        printf("%d\n", s_sum[i]);

    }
    //加了一个等待查看结果
    scanf("%s", str);

    return 0;
}