What does the error "Editor placeholder in source file" mean in C++?

Question

I am writing code to solve a knights tour problem and in my function that checks if a move is safe or not I am getting a failed build and the error is right at the top of the function:

bool KnightsTour::isValid(int x, int y, int moveNum) // error is popping up right here
{
    // grabs dimensions of board - may have mixed them up
    std::vector<std::vector<int>>::size_type xSize = board.size();
    std::vector<std::vector<int>>::size_type ySize = board[0].size();
    
    // check bounds
    if(( (0 <= x && x < xSize) && (0 <= y && y < ySize) ) && (moveNum <= totalMoves))
    {
        // in bounds and is in count limit
        // check if the sqaure has been visited
        if(board[x][y] > 0)
        {
            // has been visited
            return false;
        }
        else
        {
            // either the beginning or has not been visited
            return true;
        }
    }
    
    // move not valid
    return false;
}

I feel like my logic is right, and even if it wasn't the build wouldn't fail. In my class definition I defined the function as this:

class KnightsTour
{
private:
    int totalMoves;
    std::vector<std::vector<int>> board;
public:
    KnightsTour(int, int);
    bool isValid(int, int, int);
    // erased other methods for clarity
};

I have never seen this error before, and I don't see the issue personally. Any help is appreciated thank you.