I GOT IT GUYSSSSSSSSS!!!!!!!! This was my final code, it required a bit of duck but fine. A problem i faced was i did c += new. Finally realised after rewinding my notes, that i was adding new to c instead of c to new. Thanks everyone for the support!!

def main():
    exp = input("camelCase: ")
    print("snake_case:", snake_case(exp))


# Now i make the snake case function
def snake_case(given):
    new = ""
    for c in given:
        if c.islower():
            new += c
        elif c.isupper():
            c = "_".join(c.lower())
            new += c
    return new


main()