[Answered ]-Django custom command complains AssertionError: ForeignKey(None) is invalid. why

Your class Units should come before your class Salarys:

class Units(models.Model):
    ...

class Salarys(models.Model):
    user = models.ForeignKey(Users, verbose_name = u'account', on_delete = models.PROTECT) 
    unit = models.ForeignKey(Units, verbose_name = u'def_unit', on_delete = models.PROTECT, null=True)

One more recommendation: it’s a best practice to name your model in singular. Django will automatically “pluralize” them. If Django fails to pluralize the class name properly, you can specify your own plural by adding the following to the models Meta:

class Meta:
    verbose_name_plural = "salaries"