Making a static map with pair as a key [duplicate]

Trying to solve a leetcode problem. Here's the link. In the problem I am trying to make a static map with pair as a key.

As I removed the static keyword, the code is working perfectly fine.

But in case of static keyword, Getting a weird error message like.

In file included from prog_joined.cpp:1:
In file included from ./precompiled/headers.h:34:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/algorithm:71:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/pstl/glue_algorithm_defs.h:13:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/functional:61:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/unordered_map:46:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable.h:35:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable_policy.h:1382:2: error: static_assert failed due to requirement 'std::__is_invocable<const std::hash<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>>> &, const std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>> &>{}' "hash function must be invocable with an argument of key type"
        static_assert(__is_invocable<const _H1&, const _Key&>{},
        ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/hashtable.h:1417:34: note: in instantiation of member function 'std::__detail::_Hash_code_base<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>, std::pair<const std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>, std::pair<int, int>>, std::__detail::_Select1st, std::hash<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>::_M_hash_code' requested here
      __hash_code __code = this->_M_hash_code(__k);
                                 ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/unordered_map.h:921:21: note: in instantiation of member function 'std::_Hashtable<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>, std::pair<const std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>, std::pair<int, int>>, std::allocator<std::pair<const std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>, std::pair<int, int>>>, std::__detail::_Select1st, std::equal_to<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>>, std::hash<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true>>::find' requested here
      { return _M_h.find(__x); }
                    ^
Line 18: Char 19: note: in instantiation of member function 'std::unordered_map<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>, std::pair<int, int>, std::hash<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>>, std::equal_to<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>>, std::allocator<std::pair<const std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>>, std::pair<int, int>>>>::find' requested here
        if(st_end.find({id_stat[id].first,stationName})!=st_end.end()){
                  ^

MySolution :

class UndergroundSystem {
public:
    //   checkIn   map<id,pair<stationName,time>>
    static map<int,pair<string,int>> id_stat;
    
    //   map<pair<startingSt,endingSt>,pair<totalTime,count>>
    static map<pair<string,string>,pair<int,int>> st_end;
    UndergroundSystem() {
        
    }
    
    void checkIn(int id, string stationName, int t) {
        id_stat[id] = {stationName,t};
        
    }
    
    void checkOut(int id, string stationName, int t) {
        if(st_end.find({id_stat[id].first,stationName})!=st_end.end()){
            pair<int,int> s = st_end[{id_stat[id].first,stationName}];
            st_end[{id_stat[id].first,stationName}] = {s.first+ t-id_stat[id].second,s.second+1};
        }else{
            st_end[{id_stat[id].first,stationName}] = {t-id_stat[id].second,1};
        }
        
    }
    
    double getAverageTime(string startStation, string endStation) {
        return (double)st_end[{startStation,endStation}].first/st_end[{startStation,endStation}].second;
    }
};

/**
 * Your UndergroundSystem object will be instantiated and called as such:
 * UndergroundSystem* obj = new UndergroundSystem();
 * obj->checkIn(id,stationName,t);
 * obj->checkOut(id,stationName,t);
 * double param_3 = obj->getAverageTime(startStation,endStation);
 */

Can anybody please explain what is actually going on?