Trouble calling functions in main function

[RedSkeleton007]

I'm trying to learn how to call functions in a main function, going at it with the same approach as C++ (where the main function calls all other functions defined outside of it). Here's my code:

#!/usr/bin/env python3
#MainFunctionBasics.py

def definedOutsideOfMain():
    favoriteColor = input("What is your favorite color? ")
    print("Your favorite color is " + favoriteColor + ".")
    print("This function was defined outside of the main function.")
        
def main():    

    definedOutsideOfMain()
    print()    
    definedOutsideOfMainAgain()
    print()
    futureValue = calculateFutureValue(monthlyInv, yearlyInt, numYears)
    print("The future value is: " + futureValue)
    print()

    #main function definition ends here

def definedOutsideOfMainAgain():
    favoriteBeer = input("What is your favorite beer? ")
    print("Lets go drink some " + favoriteBeer + ".")
    print("This function was also defined outside of the main function.")

def calculateFutureValue(monthlyInvestment, yearlyInterest, years):
    monthlyInvestment = float(input("Enter the monthly investment: "))
    yearlyInterest = float(input("Enter the yearly interest rate: "))
    years = int(input("Enter the number of years: "))
    #convert yearly values to monthly values:
    monthlyInterest = yearlyInterest / 12 #/ monthlyInvestment - wtf is this?
    months = years * 12
    #calculate future value:
    futureValue = 0.0
    for i in range(0, months):#display starting at 0
        futureValue += monthlyInvestment
        monthlyInterest = futureValue * monthlyInterest
        futureValue += monthlyInterest
    return futureValue    
    
main()

The errors are:

Traceback (most recent call last):
  File "I:/Python/Python36-32/SamsPrograms/MainFunctionBasics.py", line 41, in <module>
    main()
  File "I:/Python/Python36-32/SamsPrograms/MainFunctionBasics.py", line 15, in main
    futureValue = calculateFutureValue(monthlyInv, yearlyInt, numYears)
NameError: name 'monthlyInv' is not defined

First of all, why can't I call the main function where it is right now (if that's even the problem)?
Second, I thought name miss-matches for arguments in a calling statement and in a function definition didn't matter, as long as those arguments were in the same sequence. So What is wrong with my function call in line 15?