Unresolved reference 'mptt'

from django.db import models
from mptt.models import MPTTModel, TreeForeignKey


class People(MPTTModel):
    full_name = models.CharField(max_length=80, unique=True)
    position = models.CharField(max_length=80)
    employment_date = models.DateField(max_length=80)
    salary = models.DecimalField(max_digits=50, decimal_places=2)
    settle_up = models.DecimalField(max_digits=1000, decimal_places=2)
    parent = TreeForeignKey('self', on_delete=models.CASCADE, null=True, blank=True, related_name='children')

    def __str__(self):
        return self.full_name

    class MPTTMeta:
        order_insertion_by = ['full_name']

выдает 3 ошибки Unresolved reference 'mptt' Unresolved reference 'MPTTModel' Unresolved reference 'TreeForeignKey' не знаю как решить эту проблему