Why can’t I access my backend file that is used for routing(app.php)?

I am making a chat application using PHP on backend and React.js on frontend. However, I am encountering a problem with fetch API.

My fetch that calls an endpoint

    fetch('http://localhost/registration-processing',{
                    method:'POST',
                    headers:{
                        'Content-Type':'application/json'
                    },
                    body:JSON.stringify({
                        firstName:firstName,
                        lastName:lastName,
                        username:username,
                        email:email,
                        password:password
                    })
                })
                .then((response)=>{
                    if(response.ok){ 
                        return response.json();
                    }
                })
                .then((data)=>{
                    console.log('Response:'+data);
                })
                .catch((err)=>{
                    console.error('Error is this:'+err);
                })

I didn’t explicitly write the port in ‘localhost’ since Apache runs on port 80.Fetch worked absolutely fine before installing React, where I used plain JS. Now I get message: Access to fetch at 'http://localhost/registration-processing' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

But I tried with using mode:'no-cors' in fetch, and I get successful response(.then is executed, not catch) but result is Response:undefined,even after I tried with echo json_encode(object) to return value , because I think that app.php is unreachable, which is made for handling all routes and fetch requests.

I tried with configuring .htaccess like this, but no results

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /back-end/app.php [QSA,L]

This is my project structure:

├── back-end/
│   ├── app.php
│   └── .htaccess
├── front-end/
│   ├── react-environment(where fetch is)

So, I don’t know what could be a problem and how to fix this. Also one note: There are not problems when I use php -S localhost:8000, but errors keep showing when I use Apache server via Laragon.