Why is my friend function not working when put into a separate source code file in C++?

I have the following header file:

TwoDimArray.h

class TwoDimArray {
    int** data;
    int rows, cols;
public:
    TwoDimArray(int, int);
    TwoDimArray(const TwoDimArray& orig);
    virtual ~TwoDimArray();
    const TwoDimArray& operator=(const TwoDimArray &op2);
    void random();
    TwoDimArray operator+(TwoDimArray &op2);
    TwoDimArray operator+(const int &in);
    TwoDimArray operator*(TwoDimArray &op2);
    int* operator[](int i);
    friend ostream& operator<<(ostream &os, TwoDimArray &twoDimArray);
};

And I define the friend function in my .cpp file as follows:

TwoDimArray.cpp snippet

ostream& TwoDimArray::operator<<(ostream &os, TwoDimArray &twoDimArray)
{
    for (int i = 0; i < twoDimArray.rows; i++)
    {
        os << "{";
        for (int j = 0; j < twoDimArray.cols; j++)
        {
            os << twoDimArray.data[i][j];
             if (j < twoDimArray.cols - 1)
            {
                os << ", ";
            }
        }
        os << "}";
        if (i < twoDimArray.rows - 1)
        {
            os << ", ";
        }
     }
   os << endl;
    return os;
}

For some reason, I get the following error message: TwoDimArray.cpp:171:10: error: 'std::ostream& TwoDimArray::operator<<(std::ostream&, TwoDimArray&)' must have exactly one argument

After some research I found varying answers with some people referring to the fact that "this" is implied as well as the fact that you can’t define friend functions within classes.

But this does not make sense to me because originally, there was only a .cpp file with the main method and the TwoDimArray class with no header file. And the friend function was defined within that class and there was no such error:

So it looks like it is possible to have a friend function defined within a class, but something weird happens when you put a declaration in a header file.

ostream& TwoDimArray::operator<<(ostream &os, TwoDimArray &twoDimArray) {

ostream& operator<<(ostream &os, TwoDimArray &twoDimArray) {