Decide if string is a palindrome using 3 functions

I'm supposed to make a code that determines if entered string is a palindrome, the code is supposed to have 3 functions. The first is to determine if string is palindrome, the second is to use isalpha() to remove all non letters and the third is to use tolower() to remove the difference between upper and lower case letters. I made all 3 separate functions but now that I have to combine them. The code just gives exception thrown.

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

int isPalindrome(char inputString[])
{
    int middle = strlen(inputString) / 2;
    int len = strlen(inputString);
    for (int i = 0; i < middle; i++)
        if (inputString[i] != inputString[len - i - 1])
        {
            return 0;
        }
        else
        {
            return 1;
        }
}

void checker(char stringWithoutSpace[], char bigArray[]) 
{
    int j = 0;
    for (int i = 0; i < strlen(stringWithoutSpace); i++)
    {
        if (isalpha(stringWithoutSpace[i]) != 0)
        {
            bigArray[j++] = stringWithoutSpace[i];
        }
    }
} 

void lowerCase(char* string)
{
    int i;
    for (i = 0; string[i]; i++) 
    {
        string[i] = tolower(string[i]);
    }
}

#define SIZE 1000

int main(void) {
    int repeat = 1;
    char arrayPalindrome[SIZE];
    char bigArray[SIZE];
  
    while (repeat == 1)
    {   
        printf("Enter a sentence: ");
        scanf_s("%s", &arrayPalindrome);

        checker(arrayPalindrome, bigArray);
        lowerCase(arrayPalindrome, bigArray);

        if (isPalindrome(arrayPalindrome) == 1) 
        {
            printf("This is a palindrome.");
        }
        else
        {
            printf("this in not a palindrome: ");
        }
        printf("Do you want to enter another sentence (0 for no, 1 for yes)?");
        scanf_s("%d", &repeat);
    }

    return 0;
}

For starters this call of scanf_s is incorrect.

scanf_s("%s", &arrayPalindrome);

You have to write

scanf_s("%s", arrayPalindrome, rsize_t( sizeof( arrayPalindrome ) ) );

The function checker does not build a string in the array bigArray because you forgot to append the stored sequence of characters in the array with the terminating zero character '\0'.

Moreover the function does not make sense because the array bigArray is not used anymore in the program.

The function lowerCase is declared with one parameter

void lowerCase(char* string);

but is called with two arguments.

lowerCase(arrayPalindrome, bigArray);

Pay attention to that there is no need to declare the auxiliary array bigArry.

And the program should not change the original string.

The program can look for example the following way

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

int lowerCase( char c )
{
    return tolower( c );
}

int checker( char c )
{
    return isalpha( ( unsigned char )c ) != 0;
}


int isPalindrome( const char s[] )
{
    int is_palindrome = 1;

    for (const char *first = s, *last = s + strlen( s ); is_palindrome && first < last; ++first)
    {
        while (first < last && !checker( *first )) ++first;
        if (first != last)
        {
            while (--last != first && !checker( *last ));
        }

        is_palindrome = first == last || lowerCase( *first ) == lowerCase( *last );
    }

    return is_palindrome;
}

int main( void )
{
    const char *s = "12A b B a345";

    printf( "\"%s\" is %spalindrome\n", s, isPalindrome( s ) ? "" : "not " );
}

The program output is

"12A b B a345" is palindrome