Exception in thread "main" java.lang.ClassCastException: class reactor.core.publisher.FluxPeekFuseable cannot be cast to class java.lang.String

I am learning the Spring Cloud Function. Why should we learn this? I am below error when running the code

Exception in thread "main" java.lang.ClassCastException: class reactor.core.publisher.FluxPeekFuseable cannot be cast to class java.lang.String (reactor.core.publisher.FluxPeekFuseable is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
    at com.course.kafkaproducer.SpringCloudFunctionApplication.main(SpringCloudFunctionApplication.java:22)

Code

@SpringBootApplication
public class SpringCloudFunctionApplication {

    public static void main(String[] args) {
        //SpringApplication.run(SpringCloudFunctionApplication.class, args);
        
        ApplicationContext context = SpringApplication.run(SpringCloudFunctionApplication.class);
        FunctionCatalog catalog = context.getBean(FunctionCatalog.class);
        Function<String, String> function = catalog.lookup("lowercase");
        System.out.println(function.apply("hello"));
    }
    
    @Bean
    public Function<String, String> uppercase() {
        return value -> value.toUpperCase();
    }
    
    @Bean
    public Function<Flux<String>, Flux<String>> lowercase() {
        return flux -> flux.map(value -> value.toLowerCase());
    }
}