请解释我的代码有什么问题？

Question

这是我的代码

#include<stdio.h>
//program to check the validity of isbn no.
// valid isbn no if 1*d1+2*d2+3*d3+4*d4+5*d5+6*d6+7*d7+8*d8+9*d9+10*d10
//  is divisible by 11
// where d(n) is the nth digit
// the first digit can be 10 denoted by X
int main()
{
    char isbnNo[10],flag[50];
    int sum=0;
    printf("Enter the ISBN no. = ");
    scanf("%s",isbnNo);
    for(int i=0;i<10;i++)
    {
        if(isbnNo[0]=='X')
        sum=10+sum;
        sum=sum+((int)isbnNo[i]-48)*(i+1); //to convert string to int
    }
    if(sum%11==0)
    flag[100]="The ISBN no. is valid";
    else
    flag[100]="The ISBN no. is not valid";
    printf("%s",flag);
    return 0;
}

据我所知，它应该可以正常工作而不会出现任何错误。 我想知道警告的含义以及如何纠正它

    ISBNvalidity.c: In function 'main':
ISBNvalidity.c:20:14: warning: assignment to 'char' from 'char *' makes integer from pointer without a cast [-Wint-conversion]
     flag[100]="The ISBN no. is valid";
              ^
ISBNvalidity.c:22:14: warning: assignment to 'char' from 'char *' makes integer from pointer without a cast [-Wint-conversion]
     flag[100]="The ISBN no. is not valid";
              ^

输出是任何垃圾值。