What is the correct usage of the sprintf( ) and fprintf( ) function in this case?

Question

I'm creating a voting system in C. I've started off by creating a function that allows the user to register an account. One of the features of the function is it's supposed to generate a uniqueID for each new voter and assign it to the member variable uniqueID in the Voter struct. For some reason when I execute the code it adds every other variable into the voterDatabase.txt file except for the uniqueID. Why is this?

Update: I've updated the code, and there's slight progression. It writes most of the struct member variables in cleartext, except for the uniqueID which is written as garbage. I've also added added comments for the non-standard conio.h stuff so people can follow the code better.

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

//Forward declaration
struct Voter;

//Function prototypes
int calcExistingVoters();
void createAccount(struct Voter** voterPtr, int* numVoters);

//Voter struct
struct Voter {
    char firstName[20];
    char lastName[20];
    char email[60];
    char password[17];
    int age;
    char vote[20];
    char uniqueID[8];
};

int main(){
    struct Voter* voterPtr = malloc(sizeof(struct Voter));
    int numVoters = calcExistingVoters();
    createAccount(&voterPtr, &numVoters);
    free(voterPtr);
}

int calcExistingVoters(){
    int numVotersInFile;
    FILE *acc;
    acc = fopen("voterDatabase.txt", "r");
    if (acc != NULL) {
        // read the number of voters from the file
        fseek(acc, 0, SEEK_END); //Seeks to the end of the file
        long int fileSize = ftell(acc); //calculates file size in bytes
        numVotersInFile = fileSize / sizeof(struct Voter);// calculates number of existing voters 
        fclose(acc);
    }
    return numVotersInFile;
}

void createAccount(struct Voter** voterPtr, int* numVoters){
    FILE *acc; //initializes file pointer
    char finish;
    acc = fopen("voterDatabase.txt", "a+"); //opens file in append mode
    if (acc == 0) { //if file fails to open, creates the file
        acc = fopen("voterDatabase.txt", "w+");
        system("cls");
        printf("Please wait one moment while we create the voter database on your system\n");
        printf("Complete! Press any key to continue\n");
        getch();
        fclose(acc);
        acc = fopen("voterDatabase.txt", "a+");
    }
    while(1){ //Infinite loop
        printf("Enter your first name: ");
        scanf("%s", (*voterPtr)->firstName);
        printf("\nEnter last name: ");
        scanf("%s", (*voterPtr)->lastName);
        printf("\nEnter your age: ");
        scanf("%d", &(*voterPtr)->age);
        printf("\nEnter your email: ");
        scanf("%s", (*voterPtr)->email);

        //creates password
        printf("\nCreate a password (Max. 16 Characters): ");
        char pword[17];
        for (int i = 0; i < 16; i++) {//for loop allows user to type a max of 16 chars
            pword[i] = getch(); //takes user input and assigns it to pword string
            char c = pword[i]; //assigns pword string to new char variable
            if(c==13) break; //when user presses enter key the for loop exits
            else printf("*"); //prints an asterix instead of input character to simulate password entry
        }

        pword[16] = '\0'; //assigns null character pword string
        strcpy((*voterPtr)->password, pword); //copies pword string into Voter member variable    
        printf("\nAccount creation complete, press esc to go back to the Main menu");

        // Assign a unique ID to the voter
        (*numVoters)++; //increments number of voters 
        char voterId[8]; //Initializes voterId variable
        sprintf(voterId, "V%d", (*numVoters)); //prints voterId to file
        strcpy((*voterPtr)->uniqueID, voterId);
        strcpy((*voterPtr)->vote, "");
        
        finish = getch(); // take user input and assign to "finish" variable
        if (finish == 27) //when user presses esc key it exits loop
            break;
    }

    //writes new data to file
    fprintf(acc, " %s\n %s\n %d\n %s\n %s\n %s\n %s\n",
            (*voterPtr)->firstName, (*voterPtr)->lastName, (*voterPtr)->age,
            (*voterPtr)->email, (*voterPtr)->password, (*voterPtr)->vote,
            (*voterPtr)->uniqueID); 
    fclose(acc);
}