Error 405 Method Not Allowed

So basically no matter what I do I still get the same error over and over which is 405 Method Not Allowed

document.addEventListener("DOMContentLoaded", function () {
    const submitButton = document.getElementById("submit-button");
    const messageElement = document.getElementById("message");

    submitButton.addEventListener("click", function () {
        console.log("Button clicked"); // Add this line
        const usernameInput = document.getElementById("username-input");
        const keyInput = document.getElementById("key-input");
        const passwordInput = document.getElementById("password-input");
        const username = usernameInput.value;
        const key = keyInput.value;
        const password = passwordInput.value;

        // Make an AJAX request to db.php using a POST request
        const xhr = new XMLHttpRequest();
        xhr.open("POST", "../includes/db.php", true);
        xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        xhr.onreadystatechange = function () {
            if (xhr.readyState === 4) {
                if (xhr.status === 200) {
                    const response = xhr.responseText;
                    if (response === 'UserLoggedIn') {
                        // Key is valid, user logged in
                        messageElement.innerText = 'Logged in successfully!';
                    } else {
                        // Some error occurred
                        messageElement.innerText = 'Error: ' + response;
                    }
                } else {
                    // Handle HTTP error
                    messageElement.innerText = 'HTTP Error: ' + xhr.status;
                }
            }
        };

        // Send the data to db.php
        xhr.send(`username=${encodeURIComponent(username)}&key=${encodeURIComponent(key)}&password=${encodeURIComponent(password)}`);
    });
});

It is this line

xhr.send(`username=${encodeURIComponent(username)}&key=${encodeURIComponent(key)}&password=${encodeURIComponent(password)}`);

and ik it's javascript
I tried using fetch
aswell
literally anything I do it gives me that error