1.您需要将sum和length初始化为0。否则，它们将保留垃圾值
1.接受输入的循环不正确。您需要

do {

...} while (x <= 0);

否则将无限循环。
1.您需要将这两条线互换，使其变为：

int digit = (int) (x % 10);
x = x/10;

否则，在第i次迭代中，digit将获得第i+1位，而不是第i位。

1. else (...)是无效语法。您需要else if (...)。
   完整列表：

#include <stdio.h>
int main(void)
{
  long long cc_number;
  long long x;
  long long length = 0;
  int sum = 0, count = 0;

  cc_number = (x = 54560);

  while (x != 0)
  {
    x= x/10;
    length++;
  }

  x= cc_number;

  while (x != 0)
  {
    x= x/10;
    int digit= (int) (x % 10);
    count++;
    if ((count % 2 == 0) && (length %2 ==1))
    {
      sum=sum+digit;
    }
    else if((count % 2 == 1) && (length %2 ==0))
    {
      sum=sum+digit;
    }
  }

  printf("The sum is %i\n", sum);
  printf("The length of the digits is %lli\n", length);
}

这个打印出来

$ ./a.out   
The sum is 10
The length of the digits is 5

（我对print语句做了一些修改。）

#include <cs50.h>
#include <stdio.h>

int main(void)

{
    long long length = 0;
    int sum = 0, count = 0;
    long long cc_number;
    long long a = get_long("Please Enter Credit Card Number ");

    cc_number = a;

  while (a != 0)
  {
    a= a/10;
    length++;
  }

  a = cc_number;

  while (a != 0)
{
    int digit= a % 10;
    a= a/10;
    count++;

    if (count % 2 == 0)
    {
      sum=sum+digit;
    }
}

  if (sum % 2 == 0)
  {
    printf("СС Number %lli\n", cc_number);
  }

  if ((count == 15) && (cc_number / 10000000000000 == 37 || cc_number / 10000000000000 == 34  ))
  {
    printf("American Express\n");
  }

  if ((count == 16) && (cc_number / 100000000000000 == 51))
  {
    printf("MarsterCard\n");
  }

  else
  {
    printf("INVALID\n");
  }
}