Jquery AJAX POST is returning ERROR 500 - Google Text to Speech Module

I have a text to speech module made using google's API, However, it used to work fine (converting user-entered text to speech). But since yesterday I come across this error at Jquery Ajax POST showing error 500 in the console.

The Code goes as : Index Page code:

//On click of speak button after adding text in input fields

$('#speak').click(function (e) {

        let _speakBtn = $(this);
        // prevent double click
        if (_speakBtn.attr('disabled')){
            return false;
        }
        _speakBtn.attr('disabled','disabled');

        // validate is content not empty
        if ($("#content").val().length < 1 || $.trim($("#content").val()) === ''){
            $("#content").addClass('is-invalid');
            _speakBtn.prop('disabled',false);
            return false;

        }else{
            $("#content").removeClass('is-invalid');
        }

        // validate is language not empty
        if ($("#language").val().length !== 5 || $.trim($("#language").val()) === ''){
            $("#language").addClass('is-invalid');
            _speakBtn.prop('disabled',false);
            return false;

        }else{
            $("#language").removeClass('is-invalid');
        }

        // validate is voice quality not empty
        if (($("#voice_quality").val() !== 'WaveNet' && $("#voice_quality").val() !== 'Standard') || $.trim($("#voice_quality").val()) === ''){
            $("#voice_quality").addClass('is-invalid');
            _speakBtn.prop('disabled',false);
            return false;

        }else{
            $("#voice_quality").removeClass('is-invalid');
        }

        // validate is gender not empty
        if (($("#gender").val() !== 'Female' && $("#gender").val() !== 'Male') || $.trim($("#gender").val()) === ''){
            $("#gender").addClass('is-invalid');
            _speakBtn.prop('disabled',false);
            return false;

        }else{
            $("#gender").removeClass('is-invalid');
        }

        // validate is artist not empty
        if ($("#artist_name").val().length < 2 || $.trim($("#artist_name").val()) === ''){
            $("#artist_name").addClass('is-invalid');
            _speakBtn.prop('disabled',false);
            return false;

        }else{
            $("#artist_name").removeClass('is-invalid');
        }

        $.post('<?= rtrim(TTS_APP_BASE_URL,'/') ?>/ajax.php', $('.ajax_post').serialize(), function (_response) {
            try {
                _response = JSON.parse(_response);
                if (_response.success){
                    //download audio _response.data.link
                    $('#downloadBtn').attr('data-toggle', 'modal');
                    $('#downloadForm').attr('action', _response.data.down_link);

                    //play audio
                    $('#rk_player').prop('hidden', false);
                    $('#rk_player #rk_source').attr('src', _response.data.play_link);
                    $('#rk_player').get(0).load();
                    $('#rk_player').get(0).play();
                }else{
                    alert(_response.message);
                }
            }catch (e) {
                console.error(e);
            }
            _speakBtn.prop('disabled',false);
        });
    });

In the above form the error occured at $.post('<?= rtrim(TTS_APP_BASE_URL,'/') ?>/ajax.php', $('.ajax_post').serialize(), function (_response) { line as the data is not being posted to ajax.php

The Ajax.PHP code :

/*
if($_SERVER['REQUEST_METHOD'] !== 'POST'){
    http_response_code(404);
    die();
}
*/
    define('HAS_ACCESS', TRUE);
    
    require_once 'tts_app.php';

tts_app.php code goes as:

//disable direct access to this file
defined('HAS_ACCESS') or die('silence is golden');


require 'partials/bootstrap.php';

use Classes\HandleResponseClass;
use Google\Cloud\TextToSpeech\V1\AudioConfig;
use Google\Cloud\TextToSpeech\V1\AudioEncoding;
use Google\Cloud\TextToSpeech\V1\SynthesisInput;
use Google\Cloud\TextToSpeech\V1\TextToSpeechClient;
use Google\Cloud\TextToSpeech\V1\VoiceSelectionParams;

// Authenticating with keyfile data.
/*echo "<br>";
print_r( $_ENV);
exit();*/

require_once 'config.php';

$text           = trim($_POST['text']);
$language_code  = trim($_POST['language']);
$voiceList      = TTS_APP_VOICES_LIST;
$selectLanguage = isset($voiceList[$language_code]) ? $voiceList[$language_code]:'';
$artisteName    = trim($_POST['artist_name']);
$gender         = trim($_POST['gender']);
if(empty($_POST['rate'])) {
$rate           = 1;
} else {
$rate           = $_POST['rate'];
}
if(empty($_POST['pitch'])) {
$pitch          = 1;
} else {
$pitch          = $_POST['pitch'];
}

$voice_quality  = $_POST['voice_quality'];
$u_id           = $_POST['u_id'];


// validate request
if (empty($text) || empty($selectLanguage) || empty($voice_quality) || empty($gender) || empty($artisteName) || empty($rate) || empty($pitch)){
    $response['success']    = false;
    $response['message']    = 'Required fields are empty';
    $response['data']       = [];
    echo json_encode($response);
    exit();
}

$textToSpeechClient = new TextToSpeechClient();
$input = new SynthesisInput();
$input->setText($text);
$voice = new VoiceSelectionParams();
$voice->setLanguageCode($language_code);

if (!empty($artisteName)){
    $voice->setName($artisteName);
}
if (!empty($gender)){
    $voice->setSsmlGender($gender == 'Female' ? 2:1);
}
$audioConfig = new AudioConfig();

if (!empty($rate)){
    $audioConfig->setSpeakingRate($rate);
}
if (!empty($pitch)){
    $audioConfig->setPitch($pitch);
}

$audioConfig->setAudioEncoding($voice_quality == 'WaveNet' ? AudioEncoding::OGG_OPUS:AudioEncoding::MP3);

$resp = $textToSpeechClient->synthesizeSpeech($input, $voice, $audioConfig);

// handle response file

$responseClass = new HandleResponseClass($resp);

$responseClass->saveFiles(TTS_APP_BASE_PATH);


/*=-=-= sending response -=-=-*/
$response = [
    'success'   => false,
    'message'   => 'something went wrong, please check your internet connection and try again later',
    'data'      => [],
];

if($responseClass->fileSaved()) {
    $response['success']    = true;
    $response['message']    = '';
    $response['data']       = [
        'down_link' => rtrim(TTS_APP_BASE_URL,'/').'/download.php?file='.urlencode($responseClass->getFileName()),
        'play_link' => rtrim(TTS_APP_BASE_URL,'/').'/'.$responseClass->getDirName().'/'.$responseClass->getFileName().'.mp3',
    ];
    echo json_encode($response);
    exit();
}

echo json_encode($response);
exit();

The error I m getting :

jquery-3.5.1.min.js:2          POST https://vdofy.com/tts-test/ajax.php 500
  send @ jquery-3.5.1.min.js:2
  ajax @ jquery-3.5.1.min.js:2
  S.<computed> @ jquery-3.5.1.min.js:2
  (anonymous) @ index?content_get= Test to speech tesing:1496
  dispatch @ jquery-3.5.1.min.js:2
  v.handle @ jquery-3.5.1.min.js:2

Not sure why am I getting this error as I have not changed anything since it was working fine. Please share your thoughts on what could have gone wrong.

from Jquery AJAX POST is returning ERROR 500 - Google Text to Speech Module