C++ template class error with operator '<=>' (error C2678) - c++

Question

I'm attempting to make a treap class in C++. I have the .h and .cpp files for this class, as well as a test class. My 'add' method gives me the following error when I attempt to build it: "error C2678: binary '<=>': no operator found which takes a left-hand operand of type 'E' (or there is no acceptable conversion)"
Here's my code:
Treap.h
#pragma once
#include <compare>
#include <string>
#include <random>
#include <stack>
template<class E>
class Treap
{
public:
class Node {
public:
// data fields
E data; // key for the search
int priority; // random heap priority
Node* left;
Node* right;
// omitted constructor for brevity
// omitted rotation methods for brevity
};
private:
// data fields
Node* root;
// methods
// omitted reheap method for brevity
public:
// omitted constructors for brevity
// omitted methods for brevity
};
Treap.cpp
#include "Treap.h"
template<class E>
bool Treap<E>::add(E& key, int priority) {
Node* newNode = new Node(key, priority);
if (root == nullptr) {
root = newNode;
return true;
}
else {
// if key is found in tree, return false
std::stack<Node*> nodeStack;
Node* current = root;
Node* prev = nullptr;
auto n = current->data <=> key;
nodeStack.push(current);
while (n != 0 && current != nullptr) {
prev = current;
if (n > 0)
current = current->left;
else
current = current->right;
if (current == nullptr)
break;
nodeStack.push(current);
n = current->data <=> key;
}
if (n > 0)
prev->left = newNode;
else
prev->right = newNode;
// omitted method that reheaps the treap
}
return true;
}
TreapTest.cpp
#include "pch.h"
#include "CppUnitTest.h"
#include "../Treap.cpp"
using namespace Microsoft::VisualStudio::CppUnitTestFramework;
namespace TreapTest {
TEST_CLASS(TreapTest) {
public:
TEST_METHOD(addTest) {
Treap<std::string> test;
Assert::IsTrue(test.add(std::string("p"), 99)); // this seems to be the line causing the issue
}
};
}
The lines that cause the errors are lines 13 and 26 (aka the lines with the spaceship operator).
I'm using VS 2019 and the C++ language standard is C++20.
P.S.: I'm a newbie to C++. I'm converting my existing Java code to C++ as an exercise to understand and learn C++.
Edit: OK, I made some of the changes suggested by #cdhowie and #interjay. But none of those suggestions helped make this specific issue go away (which I somewhat expected since the code compiles without the test file in the solution, meaning that the original code compiles just fine despite my dumb mistakes).
Also, #Barry, I hope these updates better suit that standard.