Why doesn't this c program work?

This program is designed to print a Fortune Cookie message for a positive integer entered by the user.

Code:

int sum_digits(int);
void print_cookie(int);

int main(void)
{
    int number, // input
                sum;        // the sum of the digits for the number

    printf("Enter a positive integer: ");
    scanf("%d", &number);
    
        sum= sum_digits(number);

        print_cookie(sum);

    return 0;
}

int sum_digits(int number)
{
        int remainder, sum_int;

        while(number!=0 && remainder!=0)
        {
                sum_int= sum_int+ number- number/10*10;
                number= number/10;
                remainder=sum_int%10;

                if(number==0 && remainder!=0)
                {
                        number=sum_int;
                        sum_int=0;
                }
        }
        return sum_int;
}

void print_cookie(int sum)
{
        switch(sum)
        {
                case 1:
                printf("You will have a fine capacity for the enjoyment of life.\n");
                break;
                case 2:
                        printf("Now is the time to try something new.\n");
                break;
                case 3:
                        printf("Don't let doubt and suspicion bar your progress.\n");
                break;
                case 4:
                printf("Your principles mean more to you than any money or success.\n");
                break;
                case 5:
                printf("Accept the next proposition you hear.\n");
                break;
                case 6:
                printf("A handful of patience is worth more than a bushel of brains.\n");
                break;
                case 7:
                printf("You have an active mind and a keen imagination.\n");
                break;
                case 8:
                printf("You are talented in many ways.\n");
                break;
                case 9:
                printf("Treat everyone as a friend.\n");
                        break;
        }
}

The function of the function sum_digits is to calculate the sum of the digits of the input number and the sum must be a single digit since the fortune numbers are single digited so if the sum exceeds one digit, add them together

e.g.
input number is 12345
sum= 1+2+3+4+5
sum= 15
sum= 1+5
sum= 6