I have error like in title when I'm trying to run test, I dont know whats going on but my testUser doesn't work properly, It's funny because i have identical test user in another project and there everything is ok.

test_api.py

class TaskDetailViewAPI(APITestCase):
 def setUp(self):
 self.client = APIClient()
 self.user = User.objects.create_user(username='test', password='test123')
 self.user.save()

 @classmethod
 def setUpTestData(cls):
 user = User.objects.get(id=1)
 Task.objects.create(name='TestTask', user=user, status='NEW', date=date(2019, 4, 9), description='This is test')

 def test_access_to_view_logged(self):
 task= Task.objects.get(id=1)
 login = self.client.login(username='test', password='test123')
 self.assertTrue(login)

And this is test from another project where everything works fine

class CreateCommentAPI(APITestCase):
 def setUp(self):
 self.client = APIClient()
 self.user = User.objects.create_user(username='test', password='test123')
 self.user.save()

 @classmethod
 def setUpTestData(cls):
 Category.objects.create(name='PC', slug='pc')
 Product.objects.create(
 category=Category.objects.get(id=1),
 name='Laptop', slug='laptop',
 description='here is description',
 photo=SimpleUploadedFile("file.jpeg", b"file_content", content_type="image/jpeg"),
 price=1999, available='available'
 )

 def test_access_to_view_logged(self):
 product = Product.objects.get(id=1)
 login = self.client.login(username='test', password='test123')
 response = self.client.get(reverse('add_comments', kwargs='id': product.id))
 self.assertTrue(login)
 self.assertEqual(response.status_code, 200, f'expected Response code 200, instead get response.status_code')

setUpTestData is called only once for the whole test class, but more importantly it is called before setUp.

Your working code doesn't have anything inside setUpTestData that depends on data in setUp, which is correct. But your non-working code does; it tries to access the User, which hasn't been created yet. You need to refactor things so that the User is either created inside setUpTestData, or the Task is created inside setUp.

Your test user's id might not be 1, instead of using the id, you could use the username in your setUpTestData method:

user = User.objects.get(username='test')