After while(*sptr1!='\0')... sptr points to the null-terminator of the string and then you are switching this null terminator with the first character. E.g. you move the null terminator to index 0. You have to decrement sptr before starting the reverse.

You should also decrement len by 2, otherwise you would iterate over the whole array and switch the already switched characters back.

Some other small mistakes:
main should return int, not void.
scanf("%s", &str1); should be scanf("%s", str1);, str1 already decays to a pointer.
You should add \n in your printf statements to have the output in different lines instead of 1 long line.

#include<stdio.h>

int main() {
    char str1[10];
    char temp;
    
    char *sptr1;
    char *sptr2;
    
    int len;
    printf("Enter a string:\n");
    scanf("%s", str1);
    sptr1 = str1;
    sptr2 = str1;
    
    while ( *sptr1 != '\0') {
        sptr1++;
    }
    
    len = sptr1 - str1;
    printf("Length of the string:%d\n", len);
    sptr1--;
    
    while (len > 0) {
        temp = *sptr1;
        *sptr1 = *sptr2;
        *sptr2 = temp;

        sptr1--;
        sptr2++;
        len = len-2;
    }
    printf("%s\n", str1);
}

See it live: https://ideone.com/WAnQLi