Cannot write on json file in express app nodejs - node.js

Question

I'm trying to read and write some data in express application (in a json file which already exists and has some data) using FS nodejs module but it returns this error in console:
Object { errno: -13, code: "EACCES", syscall: "open", path: "data.json" }
It works in localhost, but not on my web server and I don't understand where is the problem.
I've tried several ways to write on file and I'm sure this one works perfectly.
SERVER SIDE CODE:
app.post('/addLocation', function(req, res){
var jsonFile = JSON.parse(fileSystem.readFileSync(path.join(__dirname,'data.json'),'utf8'));
jsonFile.locations.push(req.body); //json data is an array named 'locations'
fileSystem.writeFile('data.json', JSON.stringify(jsonFile, null, 2), function(err){
if (err) res.send(err);
else res.send('SERVER MESSAGE: data saved');
});
})
CLIENT SIDE CODE:
const data = {name, address, lat, lng, content, audience, detail, language, purpose, description};
$.ajax({
url: '/addLocation',
type: "POST",
data: data,
success: function(result){
console.log(result);
}
});
ERROR in console:
Object { errno: -13, code: "EACCES", syscall: "open", path: "data.json" }