PHP - OAU-251 error - Unauthorized for this grant type

I have read through the previous posts concerning this subject, but have not found a solution that works for my project. My app settings are set to the JWT auth flow, and I have double-checked my credentials. I was able to connect to the API using those credentials with an Ajax call in node.js, but now we are pivoting to using a PHP backend to make the calls.

I have set up the JWT auth call following the steps in the developer documentation, with the RingCentral PHP SDK installed; however, whenever I run the code, it serves the OAU-251 "Unauthorized for this grant type" error. What am I missing?

Here is my code.

The time variables in the function are to be used later and are not used in the auth request; I have commented them out while testing to ensure those are not somehow causing errors in the auth portion.

I have also tried using both the encoded and unencoded version of the client_id and client_secret.

<?php

// Load required files and classes
require 'vendor/autoload.php';
use RingCentralSDKSDK;

// Load environmental file
$dotenv = DotenvDotenv::createImmutable(__DIR__);
$dotenv->load();

// Get environmental variables
$client_id = $_ENV['CLIENT_ID'];
$client_secret = $_ENV['CLIENT_SECRET'];
$rc_url = $_ENV['RC_SERVER_URL'];
$api_key = $_ENV['API_KEY'];

function dataRefresh($client_id, $client_secret, $rc_url) {

    // Set time variables for requests
    $start = substr(date("c", strtotime("- 1 minute")), 0, -9);
    $end = substr(date("c"), 0, -9);
    
    // Base 64 encode client_id and client_secret
    $encoded_id = base64_encode($client_id);
    $encoded_secret = base64_encode($client_secret);

    // JWT Authorization
    $rcsdk = new RingCentralSDKSDK( $client_id, $client_secret, $rc_url );
    $platform = $rcsdk->platform();
    $platform->login( [ "jwt" => $_ENV['RC_JWT'] ] );

    // Make access token available in scope
    $access_token = $_ENV['RC_JWT'];
    //echo $access_token;
} 

dataRefresh($client_id, $client_secret, $rc_url);