The problem is that you are using the @WebMvcTest annotation without making the origin of the Index Service Bean explicit. When using this annotation, Spring Boot loads the application in a context to include only components related to MVC, however, this configuration does not load beans that are not directly linked to MVC.

As you can see in the documentation, it is highly recommended to use this testing configuration in conjunction with @MockBean or @Import, depending on your needs.

Using MockBean, you can mock your Bean and within the test create isolated behavior for calling some function:

@MockBean
private IndexService indexService;

Then inside of your test:

when(indexService.createIndex(any(CreationInputDto.class))).thenReturn(/*appropriate response*/);

With @Import, you can simply import the Bean in question into your test context using the annotation in your class.

@Import(IndexService.class)