When you enter a URL in the browser's address bar, it sends a GET request. Since copying the URL in the browser tab works, it indicates that your route is set up as a GET route.

However, SSLCommerz uses a POST method for redirection, so you need to provide a URL that accepts POST requests as your callback URL.

Ideally, you should set your backend URL as the callback URL, handle the business logic in the corresponding method, and then redirect to your frontend from there.