Pointer to Pointer to Char Array

My function passwords takes char** as input. I need the function to place a specific char at a location. My program crashes and I'm not sure what I'm doing wrong but I've narrowed it down to the pointer:

// The function
int passwords(int num, int line, char** letters, char** ptrPassword, int length) {
    char* password = *ptrPassword;

    // Later in the code
    password[location] = letters[line][0];
}

Here is my call from main:

char** password;
password = malloc(length * sizeof(char));
location = length;

//call function
printf("Password at %d is %s \n", rank, passwords(rank, 0, letters, password, length));

I'm not very experience with pointers, could someone please assist?

Main:

#include<stdio.h>
#include<stdlib.h>

int passwords(int num, int line, char** letters, char** ptrPassword, int length);
int lengthOfString(char* string);
void print(char** letters, int length);

int location;

int main(){
    int numCase;
    scanf("%d", &numCase);

    int i, length;
    for(i = 0; i < numCase; i++){
        scanf("%d", &length);
        int j;
        char** letters = malloc(length*sizeof(char*));

        for(j = 0; j < length; j++){
            letters[j] = calloc(26, sizeof(char));
            scanf("%s", letters[j]);
        }
        int rank;
        scanf("%d", &rank);

        //print(letters, j);


        char* password;
        password = malloc(length * sizeof(char));
        location = length;

        //call recursion
        printf("Password at %d is %s \n", rank, passwords(rank, 0, letters, &password, length));
    }

    return 0;
}

The Entire Function:

//recursive function
int passwords(int num, int line, char** letters, char** ptrPassword, int length){
    char* password = *ptrPassword;
    printf("Recursion #%d \n", line);
    if(line == length-1){
        printf("Line is equal to length \n");
        if(num > lengthOfString(letters[line])){
            printf("IF \n");
            if(num % lengthOfString(letters[line]) == 0){
                password[location] = letters[line][lengthOfString(letters[line])];
            }
            else{
                password[location] = letters[line][num % lengthOfString(letters[line]) - 1];
            }
            printf("location: %d \n", location);
            location--;
            printf("Password is: %s \n", password);
        }
        else{
            printf("ELSE \n");
            if(num / lengthOfString(letters[line]) == 1){
                *password[location] = letters[line][0];
            }
            else{
                printf("Alocation: %d \n", location);
                password[location] = letters[line][num / lengthOfString(letters[line])];
            }
            printf("Blocation: %d \n", location);
            location--;
            printf("Password is: %s \n", password);
        }
        return lengthOfString(letters[line]);
    }
    else{
        printf("Line is not equal to length \n");
        int scalar = passwords(num, ++line, letters, ptrPassword, length);
        if (num > scalar){
            if(num % scalar == 0){
                password[location] = letters[line][lengthOfString(letters[line])];
            }
            else{
                password[location] = letters[line][num % scalar - 1];
            }
            location--;
        }
        else{
            if(num / scalar == 1){
                password[location] = letters[line][0];
            }
            else{
                password[location] = letters[line][num / lengthOfString(letters[line])];
            }
            location--;
        }
        return scalar * lengthOfString(letters[line]);
    }
}