#include<stdio.h>
#define N 40
int Readscore(int score[]);
void Datasort(int score[],int n);
void printscore(int score[],int n);
int main()
{
    int score[N],n;
    n=Readscore(score);
    printf("total students are %d\n",n);
    Datasort(score,n);
    printf("sorted scores:");
    printscore(score,n);
    return 0;
}
int Readscore(int score[])
{
    int i=-1;
    do{
        i++;
        printf("input score:");
        scanf("%d",&score[i]);
    }while(score[i]>=0);
    return i;
}
    void Datasort(int score[],int n)
    {
        int i,j,k,temp;
        for(i=0;i<n-1;i++)
        {
            k=i;
            for(j=i+1;j<n;j++)
            {
                if(score[j]>score[i])
                {
                    k=j;
                }
            }
            if(k!=i)
            {
                    temp=score[i];
                    score[i]=score[j];
                    score[j]=temp;
                }

    }
    void printscore(int score[],int n)
    {
        int i;
        for(i=0;i<n;i++)
        {
            printf("%4d",score[i]);
        }
        printf("\n");
    }

Datasort函数后面少了一个 }

另外你的排序结果好像不对，这个是算法问题，自己改吧

#include<stdio.h>
#define N 40
int Readscore(int score[]);
void Datasort(int score[],int n);
void printscore(int score[],int n);
int main()
{
    int score[N],n;
    n=Readscore(score);
    printf("total students are %d\n",n);
    Datasort(score,n);
    printf("sorted scores:");
    printscore(score,n);
    return 0;
}
int Readscore(int score[])
{
    int i=-1;
    do{
        i++;
        printf("input score:");
        scanf("%d",&score[i]);
    }while(score[i]>=0&&score[i]<=100);    //这也改了点
    return i;
}
    void Datasort(int score[],int n)
    {
        int i,j,temp;
        for(i=0;i<n-1;i++)
        {
            for(j=i+1;j<n;j++)
            {
                if(score[j]>score[i])
                {
                    temp=score[i];
                    score[i]=score[j];       //这改的有点多，不过我确实看不懂K与J的关系，我就全删了
                    score[j]=temp;
                }
            }
            

        }                                       //少个括号
    }
    void printscore(int score[],int n)
    {
        int i;
        for(i=0;i<n;i++)
        {
            printf("%4d",score[i]);
        }
        printf("\n");
    }

自己试下吧，反正我自己测试的没问题了