Undefined reference to class function in simple ToDo app

After thoroughly debugging and trying different solutions, I am super confused as to why I am getting a “undefined reference to `SaveSystem::SaveTasks(std::vector<Task, std::allocator<Task> >)' ” error in my simple ToDo app. I am not unfamiliar with programming and debugging, but am new to C++ and CLion.

I declare the variable ‘MainSave’ of custom class type ‘SaveSystem’ here in SaveSystem.h:

class SaveSystem {

    std::filesystem::path save_directory = "Saves";
    std::filesystem::path save_file = save_directory / "Save.txt";
    std::ofstream SaveFile();

    public:
    bool SaveFileExists();
    bool SaveTasks(std::vector<Task> tasks);
    bool LoadSaveFile();

    std::filesystem::path GetSaveDirectory();
    std::filesystem::path GetSaveFileName();

    SaveSystem() = default;
};

#endif //TODOCPP_SAVESYSTEM_H

inline SaveSystem MainSave;

The corresponding .cpp file “SaveSystem.cpp” looks like this:

#include "SaveSystem.h"
#include <filesystem>
#include <iostream>
#include <fstream>


bool SaveFileExists() {
    if (!std::filesystem::exists(MainSave.GetSaveDirectory())) {
        return false;
    }
    return true;
}

bool InitSaveSystem() {

    if (SaveFileExists() == false) {

        std::filesystem::create_directory(MainSave.GetSaveDirectory());
        std::cout << "New directory path created for save files." << std::endl;

        std::ofstream SaveFile(MainSave.GetSaveFileName());

    }

    else {
        std::cout << "Save directory and file already exist." << std::endl;
    }


    return true;
}

bool SaveTasks(std::vector<Task> tasks) {

    std::ofstream temp_save_file(MainSave.GetSaveFileName());

    if (temp_save_file.is_open()) {

        Task looping_task;
        for (std::vector<Task>::iterator it = tasks.begin(); it != tasks.end(); ++it) {

            looping_task = *it;

            temp_save_file << looping_task.get_task_title() << " | " << looping_task.get_task_assignee() << std::endl;
        }
    }
    else {
        return false;
    }

    return true;
}

The file and code where the error is coming from says:

/home/user/CLionProjects/ToDoCPP/main.cpp:110:(.text+0x69a): undefined reference to `SaveSystem::SaveTasks(std::vector<Task, std::allocator<Task> >)'

and main.cpp, specifically around the error:

....

    else if (UserChoice == 4) {

        MainSave.SaveTasks(ActiveTasks);

    }
}

After a couple of hours of trying to make it work, feeling like I got close, but ultimately not resolving this error, I turn to the community to please help me figure this out.

The goal of the code is to be able to save user-created tasks to a .txt file so that we can store the created tasks between user sessions.