Why is it showing '(null)' when I execute this Program?

Question

#include<stdio.h>
#include<string.h>

char *reverse(char str[])
{
    int temp;
    char val[100]="";
    for(int j=0;j<(strlen(str)/2);j++)
    {
        temp=str[j];
        str[j]=str[strlen(str)-j-1];
        str[strlen(str)-j-1]=temp;
    }
    strcat(val,str);
    return val;
}

int main()
{
    char str[20];
    
    printf("Enter a string:\n");
    scanf("%9s",str);
    
    
    printf("The Reverse is: \n%s",reverse(str));
    return 0;
}

Here is the output.

Enter a string:
Banana
The Reverse is:
(null)
[Process completed - press Enter]

This program had to take string input from a user and store the reverse of that string into a variable using a function. But when I print it, it shows '(null)'. Explanations and suggestions regarding this are appreciated.