Python是一种解释器语言，这意味着它逐行执行代码。
您的代码完全正确，但您调用此函数的位置不正确：

#call functions
if __name__ == '__main__':
    player_look = lookup_player(brave_roster)

当Python到达player_look = lookup_player(brave_roster)时，lookup_player()还没有定义。你会看到一个错误。
另外，你在main函数内部使用了brave_rosterdictionary，所以它是main函数的属性，main函数不会与其他任何人共享它的属性，因为你需要在外部使用brave_rosterdictionary ...
要解决这个问题，请将整个条件移动到程序的末尾。

我会将用户输入保存在main()中，并将名称和选择都传递给lookup_player。此外，您的return语句brave_roster(player_look)需要使用方括号来访问dict值。在下面的情况下，您将调用brave_roster[name]，但如果您想返回它，它必须在'else'中，我们知道名称在名册中：assert(name in brave_roster)。否则，您将获得KeyError。

def lookup_player(brave_roster, name, choice):
    if name not in brave_roster:
        if choice == 2:
            brave_roster[name] = "stats"
            print("Added {n} from the roster.".format(n=name))
        else:
            print("Name not in roster.")
    else:
        assert(name in brave_roster)
        if choice == 1:
            print("Here are {n}'s stats:".format(n=name))
            print(brave_roster[name])
        elif choice == 3:
            del brave_roster[name]
            print("Removed {n} from the roster.".format(n=name))
        else:
            print("N/A")
    # TypeError: 'dict' object is not callable
    # change brave_roster(player_look) to brackets
    return

def main():
    print("\t ***  Braves Stats!  ***\n")
    print("Welcome to My Braves Stats! What can I do for you?\n")
    print('  1. Search for a player')
    print('  2. Add a new player')
    print('  3. Remove a player')
    name = input("Enter the name of the player you want to look up:")
    choice = int(input("Please type your choice number:"))
    return name, choice

# call functions
if __name__ == '__main__':
    name, choice = main()
    brave_roster = {
        "Austin Riley": "AB: 615, R: 90, H: 168, HR: 38, AVG: 0.273",
        "Ronald Acuna": "AB: 467, R: 71, H: 124, HR: 15, AVG: 0.266",
    }
    player_look = lookup_player(brave_roster, name, choice)
    print("Thanks for using my Braves Stats.")