#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#include<windows.h>
 int main()
{
int hp1=100,hp2=100;
int att1,att2;
int i=0;
while(hp1>=0&&hp2>=0)
{
    att1=rand()%11+5;
    att2=rand()%11+5;
    hp2 -= att1;
    hp1 -= att2;

    printf("第%d轮，\n",i+1);
    printf("玩家一攻击力%d,玩家二剩余血量%d\n",att1,hp2);
    printf("玩家二攻击力%d,玩家一剩余血量%d\n",att2,hp1);
    if (hp2<0)printf("玩家一赢,玩家一剩余血量%d,玩家二剩余血量%d,\n",hp1,hp2);
    if(hp1 <0) printf("玩家二赢玩家一剩余血量%d,玩家二剩余血量%d,\n",hp1,hp2);
    i++;
    sleep (500);
}

return 0;
}

c(23) : warning C4013: 'sleep' undefined; assuming extern returning int

C/C++ 是区分大小写的
Windows APIs中的那个叫 Sleep

楼上正解，头文件中定义的是Sleep(),你用sleep当然会出现未定义的error