C++ newbie: Pretty printing BST using op overloading - initial value of reference to non-const must be an lvalue

I am just starting into C++ and created a binary search tree. Now I want to pretty print this tree using operator overloading for cout.

//declared in BST class
void printTree(Node* n, int indent) const;
friend ostream& operator<<(ostream& os, const BinarySearchTree& b);


void BinarySearchTree::printTree(Node * n, int indent) const
{

    if (n != nullptr)
    {
        printTree(n->left, indent + 4);
        if (indent > 0)
            cout << setw(indent) << " ";
        cout << n->data << endl;
        printTree(n->right, indent + 4);
    }

    return;

}

ostream & operator<<(ostream & os, const BinarySearchTree & b)
{
    b.printTree(b.root, 4);
}

In the b.printTree(b.root, 4) I get following errors

• initial value of reference to non-const must be an lvalue
• 'return': cannot convert from 'void' to 'std::ostream &'

Can anyone help me as to how to resolve these issue and what's the best way of going about pretty printing a BST by operator overloading in case my way is incorrect/not optimal