How to solve "error LNK2019: unresolved external symbol"?

Question

So I recently started learning C++ and a few days ago I started following a tutorial about Entity Component System, but I got stuck since I keep getting the following error:

error LNK2019: unresolved external symbol "public: void __thiscall ComponentArray<class std::bitset<8>>::insertEntity(unsigned int,class std::bitset<8>)"... referenced in function _SDL_main

I have been googling for an answer for the past few days but I just cant find a solution. I am using Visual Studio and all of the files are added to the project's solution explorer, also, I found out that if I just type everything inside of the header file and omit the cpp file it works with zero problems. Another thing is that it only gave me problems when I tried testing it inside of the main.cpp file, before that everything compiled.

ComponentArray.cpp

#include "ComponentArray.h"

template <typename T>
void ComponentArray<T>::insertEntity(Entity entity, T component)
{
    //Getting a new index and incrementing the count.
    size_t new_index = entities_created++;

    //Updating the maps to the newly added entity.
    entity_to_index[entity] = new_index;
    index_to_entity[new_index] = entity;

    //Inserting new entity.
    component_array[new_index] = component;

}

template <typename T>
void ComponentArray<T>::removeEntity(Entity entity)
{
    //Entities will be destroyed by swapping places with the last entity in array.
    //and then removing it, this way the array will always be packed.
    size_t removed_index = entity_to_index[entity];
    size_t last_index = entities_created - 1;
    Entity last_entity = index_to_entity[last_index];

    //Updating maps.
    entity_to_index[last_entity] = removed_index;
    index_to_entity[removed_index] = last_entity;

    //Cleaning maps.
    entity_to_index.erase(entity);
    index_to_entity.erase(last_index);

    //Updating array.
    component_array[removed_index] = component_array[last_index];

}

template <typename T>
void ComponentArray<T>::entityDestroyed(Entity entity)
{
    if (entity_to_index.find(entity) != entity_to_index.end())
    {
        removeEntity(entity);
    }
    else printf("The entity does not exist in the mapping.");
}

template <typename T>
T& ComponentArray<T>::getData(Entity entity)
{
    assert(entity_to_index.find(entity) != entity_to_index.end() && "The entity does not exist inside the mappings.");

    return component_array[entity_to_index[entity]];
}

ComponentArray.h

#pragma once

#include <unordered_map>
#include <assert.h>
#include <iostream>
#include <stdio.h>

#include "Types.h"

class IComponentArray
{
public:
    virtual ~IComponentArray() = default;
    virtual void entityDestroyed(Entity entity) = 0;
};

template <typename T>
class ComponentArray : public IComponentArray
{
public:
    void insertEntity(Entity entity, T component);
    void removeEntity(Entity entity);
    void entityDestroyed(Entity entity);
    T& getData(Entity entity);

private:
    //This array will keep track of the components assigned to an entity.
    std::array<T, MAX_ENTITIES> component_array;

    std::unordered_map<Entity, size_t> entity_to_index;
    std::unordered_map<size_t, Entity> index_to_entity;
 
    //total entities count = entities_created - 1 (friendly reminder)
    size_t entities_created = 0;
};

main.cpp

#include <SDL.h>
#include <SDL_image.h>
#include <stdio.h>
#include <iostream>

#include "Game.h"
#include "EntityManager.h"
#include "ComponentArray.h"


int main(int argc, char* argv[])
{
    Game* main_game = new Game("MUGEN", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600);
    
    //After adding this I started having problems.
    ComponentArray<Signature> components = ComponentArray<Signature>();

    Signature generic(1); //bitset
    components.insertEntity(0, generic);

    Signature test = components.getData(0);

    std::cout << test << std::endl;

    while (main_game->isRunning())
    {
        main_game->handleEvents();
        main_game->update();
        main_game->render();
    }

    delete main_game;

    return 0;
}