How are the names of model items not showing up in DJango Admin?

I'm building a website using python and Django, but when I looked in the admin, the names of the model items aren't showing up.

So, the objects that I am building in the admin aren't showing their names. admin.py:

from .models import Article, Author

# Register your models here.
@admin.register(Article)
class ArticleAdmin(admin.ModelAdmin):
    list_display = ['title', 'main_txt', 'date_of_publication']
    list_display_links = None
    list_editable = ['title', 'main_txt']
    def __str__(self):
        return self.title
@admin.register(Author)
class AuthorAdmin(admin.ModelAdmin):
    list_display = ['first_name', 'last_name', 'join_date', 'email', 'phone_num']
    list_display_links = ['join_date']
    list_editable = ['email', 'phone_num', ]
    def __str__(self):
        return f"{self.first_name} {self.last_name[0]}"

models.py:

# Create your models here.

class Author(models.Model):
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    date_of_birth = models.DateField()
    email = models.CharField(max_length=300)
    phone_num = models.CharField(max_length=15)
    join_date = models.DateField()
    participated_art = models.ManyToManyField('Article', blank=True)


class Article(models.Model):
    title = models.CharField(max_length=500)
    date_of_publication = models.DateField()
    creaters = models.ManyToManyField('Author', blank=False)
    main_txt = models.TextField()
    notes = models.TextField()

Add __str__() method in the model itself instead of admin.py, so:

class Author(models.Model):
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    date_of_birth = models.DateField()
    email = models.CharField(max_length=300)
    phone_num = models.CharField(max_length=15)
    join_date = models.DateField()
    participated_art = models.ManyToManyField('Article', blank=True)
    
    def __str__(self):
        return f"{self.first_name} {self.last_name}"

class Article(models.Model):
    title = models.CharField(max_length=500)
    date_of_publication = models.DateField()
    creaters = models.ManyToManyField('Author', blank=False)
    main_txt = models.TextField()
    notes = models.TextField()
    def __str__(self):
        return self.title