No match for operator== for strings

What is the meaning of error shown below?

error: no match for 'operator==' (operand types are 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' and '__gnu_cxx::__alloc_traits<std::allocator<char> >::value_type {aka char}')

This is the code I currently have:

#include<bits/stdc++.h>

using namespace std;

int main()

{

    int i , pos1=0 , pos2=0 ; 
    string trump,card1,card2;
    cin>>trump>>card1>>card2;
    string a = "6789TJQKA";
    if(trump == card1[1])
    {
        cout<<"YES";
        return 0;
    }
    for(i=0;i<a.length();++i)
    {
        if(a[i] == card1[0])
        pos1 = i;
        if(a[i] == card2[0])
        pos2 = i;
    }
    if(card1[1] == card2[1])
    {
        if(pos1>pos2)
        cout<<"YES";
        else 
        cout<<"NO";
        return 0;
    }
    else if(card1[1]!=card2[1])
    cout<<"NO";
    return 0;
}