Your issue is caused by undefined behavior due to improper memory usage:

1. Uninitialized str_of_evens → It contains garbage data, which causes strcat() to behave unpredictably
2. Using atoi() on a single character → atoi() expects a null-terminated string, but you're passing a single character
3. Buffer overflow in sum_to_str → char sum_to_str[3]; is too small for storing two-digit numbers safely

I'm attaching the corrected version:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_INPUT_LENGTH 255

int main(void) {

    char user_input[MAX_INPUT_LENGTH];
    char *p;
    printf("Welcome to the Credit Card Validator!!\n");
    printf("INSTRUCTIONS: At the prompt, please provide a CC number.\n");

    char example_card_num[] = "4003600000000014";

    int card_num_length = strlen(example_card_num);

    int skip_flag = 0;
    int sum_of_values = 0;
    char value_at_index;

    char str_of_evens[20] = {0};
   
    for (int i = card_num_length - 1; i >= 0; i--) {
        char sum_to_str[4] = {0};

        switch (skip_flag) {
            case 0:
                value_at_index = example_card_num[i];

                sum_of_values += value_at_index - '0';

                skip_flag = 1;
                break;
            case 1:
                value_at_index = example_card_num[i];
                
                int multiplied_value = (value_at_index - '0') * 2;
                
                sprintf(sum_to_str, "%d", multiplied_value);
                
                strncat(str_of_evens, sum_to_str, sizeof(str_of_evens) - strlen(str_of_evens) - 1);            
               
                skip_flag = 0;

                break;
        }
    }
    char value_at_index_two;
    for (size_t i = 0; i < strlen(str_of_evens); i++) {
        value_at_index_two = str_of_evens[i];
        sum_of_values += value_at_index_two - '0';
    }

    printf("~~~~~~~~~~~\n");

    printf("Sum of Values 01: %d\n", sum_of_values);

    return 0;
}