Resolved by adding WebMvcConfig code.

@Configuration

public class WebMvcConfig implements WebMvcConfigurer {
@Override
public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
    configurer.defaultContentType(MediaType.APPLICATION_JSON, MediaType.TEXT_PLAIN)
              .mediaType("json", MediaType.APPLICATION_JSON)
              .mediaType("text", MediaType.TEXT_PLAIN)
              .parameterName("mediaType")
              .favorPathExtension(true);
}

@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
    converters.add(new StringHttpMessageConverter(StandardCharsets.UTF_8));
    converters.add(new MappingJackson2HttpMessageConverter());
}
}