Whitelabel Error Page - Java Spring Boot Project Error

Question

Good afternoon, I’m new here, and I’m starting to study java, I’m trying to climb an initial application with spring boot, but I’m with the error Whitelabel Error Page, I tried to solve by some internet tutorials, but nothing solved, already moved the application to the beginning, already added @Springbootapplication(scanBasePackages = 'path'), but the error persists, follows below my class:

package com.evento.evento;


import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
public class IndexController {
    
    @RequestMapping("/mostrar")
    public String index() {
        return "index";//busca a pagina html
    }
}