from __future__ import unicode_literals

from django.db import models
from django.contrib.auth.models import AbstractUser
from django.utils.encoding import python_2_unicode_compatible


@python_2_unicode_compatible
class Role(models.Model):
    name = models.CharField(max_length=100)

    def __str__(self):
        return self.name


@python_2_unicode_compatible
class Category(models.Model):
    name = models.CharField(max_length=100)
    weight = models.PositiveSmallIntegerField(default=1)

    def __str__(self):
        return self.name

    class Meta:
        verbose_name_plural = "categories"
        ordering = ['weight']


class Employee(AbstractUser):
    role = models.ForeignKey(Role, null=True, blank=True)
    skype_id = models.CharField(max_length=200, null=True, blank=True)
    last_month_score = models.PositiveIntegerField(default=0)
    current_month_score = models.PositiveIntegerField(default=0)
    level = models.PositiveIntegerField(default=0)
    total_score = models.PositiveIntegerField(default=0)
    avatar = models.ImageField(upload_to='avatar', null=True, blank=True)
    categories = models.ManyToManyField(Category)

from __future__ import unicode_literals

from django.db import models
from django.contrib.auth.models import AbstractUser
from django.utils.encoding import python_2_unicode_compatible


@python_2_unicode_compatible
class Role(models.Model):
    name = models.CharField(max_length=100)

    def __str__(self):
        return self.name


@python_2_unicode_compatible
class Category(models.Model):
    name = models.CharField(max_length=100)
    weight = models.PositiveSmallIntegerField(default=1)

    def __str__(self):
        return self.name

    class Meta:
        verbose_name_plural = "categories"
        ordering = ['weight']


class Employee(AbstractUser):
    role = models.ForeignKey(Role, null=True, blank=True)
    skype_id = models.CharField(max_length=200, null=True, blank=True)
    last_month_score = models.PositiveIntegerField(default=0)
    current_month_score = models.PositiveIntegerField(default=0)
    level = models.PositiveIntegerField(default=0)
    total_score = models.PositiveIntegerField(default=0)
    avatar = models.ImageField(upload_to='avatar', null=True, blank=True)
    categories = models.ManyToManyField(Category, blank=True)

Change categories field to non required.
Change categories field to non required.
