Testing: TypeError: expected str, bytes or os.PathLike object, not NoneType

I'm testing user's creation but the test haven't reach the failed test because of the error given TypeError: expected str, bytes or os.PathLike object, not NoneType.

I have the migrations using pass on using the AbstractUser giving the following on the migrations/0001initial.py file:

# Generated by Django 3.1.4 on 2021-07-12 15:00

import django.contrib.auth.models
import django.contrib.auth.validators
from django.db import migrations, models
import django.utils.timezone


class Migration(migrations.Migration):

    initial = True

    dependencies = [
        ('auth', '0012_alter_user_first_name_max_length'),
    ]

    operations = [
        migrations.CreateModel(
            name='User',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('password', models.CharField(max_length=128, verbose_name='password')),
                ('last_login', models.DateTimeField(blank=True, null=True, verbose_name='last login')),
                ('is_superuser', models.BooleanField(default=False, help_text='Designates that this user has all permissions without explicitly assigning them.', verbose_name='superuser status')),
                ('username', models.CharField(error_messages={'unique': 'A user with that username already exists.'}, help_text='Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only.', max_length=150, unique=True, validators=[django.contrib.auth.validators.UnicodeUsernameValidator()], verbose_name='username')),
                ('first_name', models.CharField(blank=True, max_length=150, verbose_name='first name')),
                ('last_name', models.CharField(blank=True, max_length=150, verbose_name='last name')),
                ('email', models.EmailField(blank=True, max_length=254, verbose_name='email address')),
                ('is_staff', models.BooleanField(default=False, help_text='Designates whether the user can log into this admin site.', verbose_name='staff status')),
                ('is_active', models.BooleanField(default=True, help_text='Designates whether this user should be treated as active. Unselect this instead of deleting accounts.', verbose_name='active')),
                ('date_joined', models.DateTimeField(default=django.utils.timezone.now, verbose_name='date joined')),
                ('groups', models.ManyToManyField(blank=True, help_text='The groups this user belongs to. A user will get all permissions granted to each of their groups.', related_name='user_set', related_query_name='user', to='auth.Group', verbose_name='groups')),
                ('user_permissions', models.ManyToManyField(blank=True, help_text='Specific permissions for this user.', related_name='user_set', related_query_name='user', to='auth.Permission', verbose_name='user permissions')),
            ],
            options={
                'verbose_name': 'user',
                'verbose_name_plural': 'users',
                'abstract': False,
            },
            managers=[
                ('objects', django.contrib.auth.models.UserManager()),
            ],
        ),
    ]

The test case I'm using is the following:

from django.contrib.auth import get_user_model
from rest_framework import status
from rest_framework.reverse import reverse
from rest_framework.test import APITestCase

PASSWORD = 'pAssw0rd!'


class AuthenticationTest(APITestCase):
    def test_user_can_sign_up(self):
        response = self.client.post(reverse('sign_up'), data={
            'username': 'user@example.com',
            'first_name': 'Test',
            'last_name': 'User',
            'password1': PASSWORD,
            'password2': PASSWORD,
        })
        user = get_user_model().objects.last()
        self.assertEqual(status.HTTP_201_CREATED, response.status_code)
        self.assertEqual(response.data['id'], user.id)
        self.assertEqual(response.data['username'], user.username)
        self.assertEqual(response.data['first_name'], user.first_name)
        self.assertEqual(response.data['last_name'], user.last_name)

So when I run python manage.py test trips.tests, I'm getting the a traceback error: ... TypeError: can only concatenate str (not "NoneType") to str

The error was generated because I forgot to export Postgres values, like this:

$ export PGDATABASE=dbname
$ export PGUSER=dbuser
$ export PGPASSWORD=dbpassword