How to insert struct into stl::map C++

Question

I am trying to use a loop to populate a map using the insert method. I have a map that I am trying to populate using this method:

void board:: insertToMap(Letter c, int num){
    this->myRackMap.insert(pair<Letter, int>(c, num));
}

and I call this helper method in this loop within another method:

void board:: getRackAsMap(){
    for (int i = 0; i < this->getMyRack().size(); ++i){
        insertToMap(this->getMyRack().at(i), this->getMyRack().at(i).readScore());
    }
}
//myRackMap is a map<Letter, int>
//myRack is a vector<Letter>

vector<Letter>& board::getMyRack(){
    return this->myRack;
}

//readScore returns an Int value based on the char value of the current Letter

When I try to run this I get a ridiculously long error message, waaay too long to put into this. The ending line of the error message is this; however:

"‘const Letter’ is not derived from ‘const std::multimap<_Key, _Tp, _Compare, _Alloc>’ { return __x < __y; }"

which led me to believe that the error is related to inserting my struct Letter into the map instead of a primitive data type. Any advice would be greatly appreciated as I am not too familiar with c++, thank you for any help you can provide!

EDIT: here is what Letter.h looks like

struct Letter{
private:
    char theLetter;
    int xPos;
    int yPos;
public:
    Letter();
    Letter(char c);
    Letter(int x, int y, char c);
    void setPos(int x, int y);
    void setLetter(char c);
    int getXPos();
    int getYPos();
    char getTheLetter();
    int readScore();
};

and letter.cpp

Letter:: Letter(){
    this->xPos = -1;
    this->yPos = -1;
    this->theLetter = '?';
}

Letter:: Letter(char c){
    this->xPos = -1;
    this->yPos = -1;
    this->theLetter = c;
}

Letter:: Letter(int x, int y, char c){
    this->xPos = x;
    this->yPos = y;
    this->theLetter = c;
}
int Letter:: getXPos(){
    return this->xPos;
}
int Letter:: getYPos(){
    return this->yPos; 
} 
char Letter:: getTheLetter(){
    return this->theLetter;
}
void Letter:: setPos(int x, int y){
    this->xPos = x;
    this->yPos = y;
}
void Letter:: setLetter(char c){
    this->theLetter = c;
}
int Letter:: readScore(){
    switch (this->getTheLetter()){
        case 'A':
        return 1;
        break;
    case 'B':
        return 3;
        break;
    //etc etc, returns int based on char of Letter
    }
}