#include<stdio.h>
int mian()
{
    int iDay=0;
   
    printf("enter a day of week to get course:\n");
    scanf("%d",&iDay);
    switch(iDay)
    {
        case 1:
            printf("Have a meeting in the compay\n");
            break;
        case 2:
        case 3:
        case 4:
        case 5:
            printf("Working with partner\n");
            break;
        case 6:
            printf("Go shopping with frinds\n");
            break;
        case 7:
            printf("At home with families\n");
            break;
        default:
            printf("error!!\n");
    }
    return 0;
}

你要说出是什么错呀，这样别人才能有的放矢。比如“无法解析的外部符号 main，函数”
另外，这也是“运行错误”，而是“编译错误”

int mian() 改为 int main( void )

刚学c语言还没半个月，也不知道该咋问，谢谢，以后会注意的，谢谢大佬