Why I get Object references an unsaved transient instance

I have many-to-many relationship between two entities: Tag and Ads. When User wants to create ad, he need to insert few tags. But when I try to create add I am getting this error:

{
    "timestamp": "2020-10-11T21:21:44.752+0000",
    "status": 500,
    "error": "Internal Server Error",
    "message": "org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.mybrocki.entity.Tag; nested exception is java.lang.IllegalStateException: org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.mybrocki.entity.Tag",
    "path": "/mybrocki/auth/ads/create"
}

This is my Tag entity:

@Entity
@Table(name="Tags")
public class Tag {

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	private Long id;
	
	@Column(name="name")
	private String name;

	@ManyToMany(mappedBy = "adsTags",  cascade = {
	        CascadeType.ALL
	    })
	private Set<Ads> ads;
	
	
	public Long getId() {
		return id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Set<Ads> getAds() {
		return ads;
	}

	public void setAds(Set<Ads> ads) {
		this.ads = ads;
	}

And this is Ads entity:

@ManyToMany
    @JoinTable(name = "tags_ads",
        joinColumns = @JoinColumn(name = "ads_id", referencedColumnName = "id"),
        inverseJoinColumns = @JoinColumn(name = "tag_id", referencedColumnName = "id"))
	private List<Tag> adsTags;

And this is service implementation class:

ads.setVisibleAds(false);
					ads.setAdsTags(new ArrayList<Tag>());
					if (!adsDTO.getTags().isEmpty()) {
						for (Tag tagDTO : adsDTO.getTags()) {
					ads.getAdsTags().add(tagDTO); }
					

					}
					ads.setStatus(AdsStatus.READYFORREVIEW.toString());
					ads.setAdssubgroup(adsSubGroupRepository.findOneById(adsDTO.getAdsSubGroupId()));
					ads.setFixedPrice(adsDTO.getFixedPrice());
					ads.setFreeDelivery(adsDTO.getFreeDelivery());
					ads.setProductWarranty(adsDTO.getProductWarranty());
					ads.setUrgentSales(adsDTO.getUrgentSales());
					Date currentDate = new Date();
					Instant instant = currentDate.toInstant();
					LocalDateTime ldt = instant.atOffset(ZoneOffset.UTC).toLocalDateTime();
					DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
					
					adsDTO.setAdsDate(currentDate);
					ads.setAdsDate(adsDTO.getAdsDate());
					
					adsRepository.save(ads);

This is json:

{
  "adsGroupId": 5,
  "adsSubGroupId": 27,
  "adsType": "USED",
  "description": "Ford fiesta 1.3",
  "fixedPrice": false,
  "freeDelivery": false,
  "image": [
    "http://res.cloudinary.com/mybrocki/image/upload/v1589887253/s5d11rshydka5nov2wox.jpg,http://res.cloudinary.com/mybrocki/image/upload/v1589887253/s5d11rshydka5nov2wox.jpg,"
  ],
  "price": 800,
  "productName": "FOcscscsdcRD Goragjhggiujkjhkjoijiohiuhn",
  "productWarranty": false,
  "urgentSales": true,
    "tags": [
        {
        "name":"Second hand"
    }]
}

What could be the problem?

You have cascade = {CascadeType.ALL } on your Tag class but not on your Ads class, if you want to save your Ads object and cascade save the Tag objects in it, then you need the annotation on the field of your Ads class. Otherwise save the Tag object instead of the Ads object.