Resizing vector of object of nested class inside abstract class

I am trying to build a Hex Game using the Polymorphism principle. I have a class inside an abstract class. I need a 2D vector of Cells (nested class) but I'm getting this error when I try to implement a function to resize my vector.

Base and Abstract Class = AbstractHex || Nested Class = Cell || Derived Class = HexVector

Here's what I got (ERROR):

/usr/bin/ld: /tmp/ccq9PAk3.o: in function `void std::_Construct<HexGame::AbstractHex::Cell>(HexGame::AbstractHex::Cell*)':HexVector.cpp:(.text._ZSt10_ConstructIN7HexGame11AbstractHex4CellEJEEvPT_DpOT0_[_ZSt10_ConstructIN7HexGame11AbstractHex4CellEJEEvPT_DpOT0_]+0x2d): undefined reference to `HexGame::AbstractHex::Cell::Cell()' collect2: error: ld returned 1 exit status

My Base Class (Abstract):

namespace HexGame{
   class AbstractHex{

     public:
        class Cell{
                public:
                    explicit Cell();
                    explicit Cell(int rw,int col) : row(rw) , column(col){} //INTENTIONALLY EMPTY
                    explicit Cell(int rw,int col, char p) : row(rw) , column(col) , point(p){} 
                                         //INTENTIONALLY EMPTY
                    int getRow()const {return row;};
                    int getColumn()const {return column;};
                    void setRow(int index) {row = index;};
                    void setColumn(int index) {column = index;};
                    int getPoint()const {return point;};
                    void setPoint(char param) {point = param;};    
                private:
                    int row;
                    int column;
                    char point;
            };

     virtual void setSize() = 0;

     private:
       int board_size;

I'm calling setSize() function which is in abstract class. Can I call a virtual function within a non-virtual function which is in abstract class?

void AbstractHex::startGame(){ //non-virtual function in Abstract Class
setSize(); //virtual function in Abstract Class
//I'm calling startGame() function every time I create an object of HexVector
}

My Derived Class:

class HexVector : public AbstractHex{
    public:
        void setSize() override;
    private:
        vector<vector<AbstractHex::Cell> > board;

Overriding of setSize() function

void HexVector::setSize(){
 board.resize(getBoardSize());
    for(int i=0; i<getBoardSize(); i++){
        board[i].resize(getBoardSize());
    }
}