How do I fix this error message error: no match for ‘operator[]’

I am implementing a class that involves unordered_sets, and unordered_maps. When I go to run the program it gives me the following error

error: no match for ‘operator[]’ (operand types are ‘std::unordered_set<std::__cxx11::basic_string<char> >’ and ‘const string’ {aka ‘const std::__cxx11::basic_string<char>’})
     int points = Prizes[Codes[code]]->points;

I am unsure of how to fix this, please help.

I have pasted my function (sightly condensed) below.

int Code_Processor::Enter_Code(const string &username, const string &code) {
    User *user = user_iter->second;
    int points = Prizes[Codes[code]]->points;
    user->points += points;
    Codes.erase(code);
    return points;
}

I have also included part of the header file below.

class User {
  public:
     std::string username;
     std::string realname;
     int points;
     std::set <std::string> phone_numbers;
};

class Prize {
  public:
     std::string id;
     std::string description;
     int points;
     int quantity;
};

class Code_Processor {
  public:
    int Enter_Code(const std::string &username, const std::string &code);

  protected:
    std::unordered_map <std::string, User *> Names;
    std::unordered_map <std::string, User *> Phones;
    std::unordered_set <std::string> Codes;
    std::unordered_map <std::string, Prize *> Prizes;
};