Django save User error not finding role based on choices

I've this models.py for user:

from django.contrib.auth.models import AbstractUser
from django.db.models import CharField
from django.urls import reverse
from django.utils.translation import gettext_lazy as _
from django.db import models
from django.db.utils import IntegrityError

"""
Pour toute modification des roles possibles, il faut lancer dans le terminal
python3 manage.py create_roles

Pour voir le code correspondant, il faut aller dans le repertoire management/commands/
C est une commande cree expres pour cette fonctionnalité

"""

class Role(models.Model): # si changement de roles voir ci dessus le commentaire
    ADMIN = 'ADMIN'
    SELLER = 'SELLER'
    SALES_DIRECTOR = 'SALES_DIRECTOR'
    SALES_ASSISTANT = 'SALES_ASSISTANT'
    OPERATIONS = 'OPERATIONS'
    ROLE_CHOICES = [
        (ADMIN, 'Administrator'),
        (SELLER, 'Seller'),
        (SALES_DIRECTOR, 'Sales Director'),
        (SALES_ASSISTANT, 'Sales Assistant'),
        (OPERATIONS, 'Operations'),
    ]
    role = models.CharField(
        max_length=20,
        choices=ROLE_CHOICES,
        default=SELLER,
    )

    def __str__(self):  # Here
        return self.role


class User(AbstractUser):
    """
    Default custom user model for Orphea.
    If adding fields that need to be filled at user signup,
    check forms.SignupForm and forms.SocialSignupForms accordingly.
    """

    #: First and last name do not cover name patterns around the globe
    name = CharField(_("Name of User"), blank=True, max_length=255)
    first_name = None  # type: ignore
    last_name = None  # type: ignore
    role = models.ForeignKey(Role, on_delete=models.SET_NULL, null=True)
    def get_absolute_url(self):
        """Get url for user's detail view.

        Returns:
            str: URL for user detail.

        """
        return reverse("users:detail", kwargs={"username": self.username})

    @property
    def role_choices(self):
        return [(role.id, role.role) for role in Role.objects.all()]

    def save(self, *args, **kwargs):
        # Check if role instance exists for selected role
        role, created = Role.objects.get_or_create(role=self.role)
        # Assign the role instance to the user
        self.role = role
        super().save(*args, **kwargs)

My role table contains :

pk, role  : 
6,ADMIN
7,SELLER
8,SALES_DIRECTOR
9,SALES_ASSISTANT
10,OPERATIONS

I've this error when trying to save a user :
ValueError at /admin/users/user/1/change/
Cannot assign "'ADMIN'": "User.role" must be a "Role" instance.

I don't know how to fix it.

Objectif is to maintain role choices and to check if role exists in list and then update user with role assigned. each user have only one role.

The issue was in my admin.py: I put role from choiceField but it doesn't t not fit with a role object.

class UserAdminForm(forms.ModelForm):

    #role = forms.ChoiceField(choices=Role.ROLE_CHOICES)
    role = Role.objects.all()

    class Meta:
        model = get_user_model()
        fields = '__all__'