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string key = argv[1];
int length = strlen(key);

下面

while (argc != 2)
{
    printf("Usage: ./substituion key\n");
    return 1;
}

似乎修复了它。

首先你应该检查argc的值，如果不等于2，然后返回程序。
我认为最好使用if(argc != 2)而不是while (argc != 2)来检查argc的值，如果天气等于2。
与while (length != 26)相同，使用if(length != 26)。
与while (score != 2015)相同，使用if(score != 2015)
所以我的答案是：

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

int main(int argc, string argv[])
{
    //while (argc != 2)
    if(argc != 2)
    {
        printf("Usage: ./substituion key\n");
        return 1;
    }

    string key = argv[1];
    int length = strlen(key); // LINE WHERE DEBUGGER GETS PAUSED ON
    // prints a usage error if a user inputs no key, or more than one key

    // prints an error if a user inputs a key that isn't 26 characters
    //while (length != 26)
    if(length != 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }

    int score = 0;
    // loops through key
    for (int i = 0; i < length; i++)
    {
        // if the current character isn't a part of the alphabet, an error is printed
        if (key[i] < 'A' || key[i] > 'z')
        {
            printf("Key must contain only alphabetical letters\n");
            return 1;
        }
        // if the current character is lowercase, it is subtracted by 32 to make is uppercase, and that (ASCII) value is added to score
        if (islower(key[i]))
        {
            score += key[i] - 32;
        }
        // if the current character is uppercase, its ASCII value is added to score
        if (isupper(key[i]))
        {
            score += key[i];
        }
    }
    // if each uppercase letter of the alphabet's ASCII value are added together, it equals 2015;
    // if that added values of the key do not equal 2015, that means there is a repeated letter, which prints an error message
    //while (score != 2015)
    if(score != 2015)
    {
        printf("Key must contain each letter exactly once\n");
        return 1;
    }

    string plainText = get_string("plaintext: ");
    printf("ciphertext: ");
    for (int j = 0; plainText[j] != '\0'; j++)
    {
        char plainLetter = plainText[j];
        //printf("plain: %c\n", plainLetter);
        if (isupper(plainLetter))
        {
            printf("%c", toupper(key[plainLetter - 'A']));
        }
        if (islower(plainLetter))
        {
            printf("%c", tolower(key[plainLetter - 'a']));
        }
        if (isdigit(plainLetter) || ispunct(plainLetter) || isspace(plainLetter))
        {
            printf("%c", plainLetter);
        }
    }
    printf("\n");
    return 0;
}