main()
{     
     CARS cars[MAX];
     password();
     int sum;
     sum=read_file(cars);
     if(sum==0)
     {
         printf("please press any key to menu:");
         getch();
         sum=input(cars);                             error C2120: 'void' illegal with all types
     }
     do
          {
              int choice;
              system("cls");/*清屏*/
              printf("\n\n--★-☆--★-☆--☆-★--☆-★--☆-☆--★-\n");
              printf("\n\t\t\t    上海大众汽车管理系统\t\n\n");
              printf("\t  1 →输入汽车信息\n\n");
              printf("\t  2 →排序汽车信息\n\n");
              printf("\t  3 →统计汽车信息、\n\n");
              printf("\t  4 →增加/删除汽车信息\n\n");
              printf("\t  5 →更新汽车信息\n\n");
              printf("\t  6 →查询汽车信息\n\n");
              printf("\t  7 →浏览汽车信息\n\n");
              printf("\t  8 →打印汽车信息\n\n");
              printf(" \t\t          --☆-★--☆-★--☆-☆--★-☆--★-☆--★-");/*菜单选择界面*/
              printf("\n\n");
              printf("\t请选择您所要的操作(或选择(0)退出):");
              scanf("%d",&choice);
              switch(choice)
              {
                   case 1:input();break;       /*输入模块*/
                   case 2:sort();break;        /*排序模块*/
                   case 3:count();break;    /*统计模块*/
                   case 4:edit();break;        /*增加/删除模块*/
                   case 5:renew();break;       /*更新模块*/
                   case 6:inquire();break;     /*查询模块*/
                   case 7:scan();break;        /*浏览模块*/
                   case 8:output();break;       /*输出模块*/
                   case 0: printf("欢迎使用！！！！谢谢再见！！！！");break;          /*退出系统*/
              }
          }while(choice!=0);             'choice' : undeclared identifier
          save_file(cars,sum);
}

int choice;的声明放在循环体外面就OK了

变量的作用域，出了花括号就不认识了！