Spring Boot 3 - "Whitelabel Error Page" getting in new project

It looks like you don't have defined mappings.

e.g. src/main/java/com/example/demo/MainController.java file:

package com.example.demo;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class MainController {

    @RequestMapping(
        value = "/",
        method = RequestMethod.GET
    )
    @ResponseBody
    public String index() {
        return "Hi There!";
    }
}