return Mono.error inside Map Spring Webflux

For Matching condition, return Mono.error from code , but gives compilation error. I commented out

//return Mono.error(new RuntimeException("User Phone Exists already"));

Result:

Compilation Error: Required Type Mono<EventSlotBook>, Provided Mono<Object>

Code:

public Mono<EventSlotBook> getEventSlotBookWithAppointmentId(EventSlotBook eventSlotBookEntity) {

    Query query = new Query();
    query.addCriteria(
            new Criteria().andOperator(
                    Criteria.where("eventId").is(eventSlotBookEntity.getEventId()),
                    Criteria.where("eventConfigId").is(eventSlotBookEntity.getEventConfigId()),
                    Criteria.where("eventSlotId").is(eventSlotBookEntity.getEventSlotId())));

    return this.reactiveMongoTemplate.findOne(query, EventSlotBook.class)
            .map(eventSlotBookEntityFromDb -> {
                EventSlotBook eventSlotNewEntity = eventSlotBookEntityFromDb.toBuilder().build();

                if(eventSlotNewEntity.getEventUsers() != null) {
                    for(EventUser eventUserIno:eventSlotNewEntity.getEventUsers()) {
                        if(eventUserIno.getPhoneNumber().equalsIgnoreCase(eventSlotBookEntity.getEventUser().getPhoneNumber())){
                            //return Mono.error(new RuntimeException("User Phone Exists already"));
                        }
                    }
                }


                int maxTokenVal = Integer.valueOf(eventSlotNewEntity.getMaxTokenVal()).intValue() + 1;
                EventUser eventUser = new EventUser(eventSlotNewEntity.getEventUser().getName(),eventSlotNewEntity.getEventUser().getPhoneNumber(),String.valueOf(maxTokenVal));
                eventSlotNewEntity.getEventUsers().add(eventUser);
                eventSlotNewEntity.setMaxTokenVal(String.valueOf(maxTokenVal));
                eventSlotNewEntity.setEventUser(eventUser);


                return eventSlotNewEntity;
                //return Mono.error(new RuntimeException("Ts"));
            })
            .switchIfEmpty(getEventSlotBook(eventSlotBookEntity));
}

caller of Method : I should handle Mono.error and return to rest API that user already exists ?. Please help on this

public Mono<EventSlotBookRequestDto> saveEventSlotBook(Mono<EventSlotBookRequestDto> eventSlotBookRequestDtoMono){
    log.info("Start::SaveEventSlotBook");
    Mono<EventSlotBookRequestDto> eventDtoSaved =
            eventSlotBookRequestDtoMono.map(AppUtils::dtoToEntity)
                    .flatMap(eventSlotEntity -> getEventSlotBookWithAppointmentId(eventSlotEntity))
                    .doOnNext(eventSlotEntityBeforeSave -> {
                        log.info("@@@@BeforeSave::{}",eventSlotEntityBeforeSave);
                    })
                    .flatMap(eventSlotBookrepository::save)
                    .doOnNext( eventSlotBookAfterSave -> {
                        log.info("@@@@AfterSave::{}",eventSlotBookAfterSave);
                    })
                    .map(AppUtils::entityToDto);
    log.info("End::SaveEventSlotBook");
    return eventDtoSaved;
}