Why is this compiler error coming in my C++ project

#include <iostream>

using namespace std;

struct node
{
	int data;
	node*p_right;
	node*p_left;
};

node* insert_(node*p_tree,int key)
{
	if(p_tree==NULL)
	{
		node*p_new_tree=new node;
		p_new_tree->p_left=NULL;
		p_new_tree->p_right=NULL;
		p_tree=p_new_tree;
		return p_tree;

	}
	else if(key<p_tree->data)
	{
		p_tree->p_left=insert_(p_tree->p_left,key);
		return p_tree;
	}
	else if(key>p_tree->data)
	{
		p_tree->p_right=insert_(p_tree->p_right,key);
		return p_tree;
	}

	int print(node*p_tree)
	{
		if (p_tree==NULL)
		{
			return NULL;
		}
		else
		{
			cout <<  p_tree->data;
			print(p_tree->p_left);
			print(p_tree->p_right);

		}

	}

}

int main()
{ 
	node*p_tree=NULL;

	insert_(p_tree,4);
	insert_(p_tree,5);
	insert_(p_tree,3);
	insert_(p_tree,6);
	insert_(p_tree,2);
	insert_(p_tree,1);
	insert_(p_tree,7);
	insert_(p_tree,8);

	print(p_tree);
}