首先，这个程序将无法正常运行。除此之外，还有你的问题：
1.你必须关闭main并打开转换函数。检查所有的花括号是否正确。
1.你的函数声明在main函数里面。正如退休的忍者在评论部分所说的，这是正确的，你可以这样做。请考虑局部和全局作用域。（在main函数里面，它将成为main的局部作用域，而在声明之外，它将成为全局作用域）
但是如果你想一想，如果你在局部作用域中声明的函数下面定义另一个函数，同样的函数无论如何都会变成全局作用域。因此，最好直接在全局作用域中声明函数。现在，只是从这个Angular 来看，为了避免不必要的刺激。
1.您的函数声明和定义中有拼写错误。（conversion /convion）请检查。
1.您的函数声明并定义了一个返回值float，但您忘记了最后的return语句，您需要打印出计算结果。
。

#include <iostream> // Do you realy need all of these includes?
#include <cstdlib>
#include <iomanip>
#include <cmath>
#include <string>

using namespace std;

const int HEAD_SPACE = 23;
const int SECND_HDSPACE = 35;
const int THRD_HDSPACE = 27;
char startKey;
char replayKey;
int userChoice;

float conversion(void);

int main()
{
firstStart:
    cout << "=======================================================\n\n";
    cout << setw(SECND_HDSPACE) << "\t\t\t ::: CURRENCY CONVERTER :::\n\n";
    cout << "=======================================================\n\n";

    cout << setw(SECND_HDSPACE) << "\t\t\tPlease select any options below\n\n";

    cout << setw(SECND_HDSPACE) << "\t\t\t1) Press [1] to convert from USD to GBP\n";
    cout << setw(SECND_HDSPACE) << "\t\t\t2) Press [2] to convert from USD to EUR\n";
    cout << setw(SECND_HDSPACE) << "\t\t\t3) Press [3] to convert from USD to NZD\n";
    cout << setw(SECND_HDSPACE) << "\t\t\t4) Press [4] to convert from USD to JPY\n";

    cout << setw(SECND_HDSPACE) << "\t\t\n\n :: PRESS [S] TO START PROGRAM :: \n\n";
menuOption:
    cin >> startKey;
    if (startKey == 's' && startKey == 'S')
    {
        float resultVal = conversion();
        cout << "The conversion is: " << resultVal << endl;
        cout << "Would you like to run the program again? Press Y or N" << endl;
    scndStrt:
        cin >> replayKey;

        if (replayKey == 'y' && replayKey == 'Y')
        {
            goto firstStart;
        }
        else if ((replayKey == 'n' && replayKey == 'N'))
        {
            cout << "Thanks for using my program!" << endl;
        }
        else
        {
            cout << "Wrong Key Entered. Please hit Y or N" << endl;
            goto scndStrt;
        }
    }
    else
    {
        cout << "Wrong Key Entered. Please Hit [S]";
        goto menuOption;
    }
}

float conversion()
{

    int selection;
    int currChoice;
    float currency1;
    float currency2;
    cout << "Please enter the currency name: " << endl;
    cin >> selection;
    cout << "Please enter the amount you would like to convert:  " << endl;
    cin >> currency1;

    switch (selection) // ??
    {
    case '1':
    formulaInput:
        cout << "Please enter what currency you would like to conver in: " << endl;
        cin >> currency2; // This is not working.

        if (currency2 == '1')
        {
            currency2 = currency1 * 1;
        }
        else if (currency2 == '2')
        {
            currency2 = currency1 * 0.84;
        }
        else if (currency2 == '3')
        {
            currency2 = currency1 * 0.94;
        }
        else if (currency2 == '4')
        {
            currency2 = currency1 * 1.92;
        }
        else
        {
            cout << "Incorrect input, please try again." << endl;
            goto formulaInput;
        }
    }

    // Here you have to return something?
    return currency2;
}