Custom validation in Django admin

You have an indentation issue. Your clean method is indented within the form's Meta class. Move it back one level. Also, ensure that the return statement is indented within the method.

Usually you just want to define a clean() method on the model itself.

https://docs.djangoproject.com/en/2.1/ref/models/instances/#validating-objects

from django.core.exceptions import ValidationError

class Lecture(models.Model):
    topic = models.CharField(max_length=100)
    speaker = models.CharField(max_length=100)
    start_date = models.DateField()
    end_date = models.DateField()

    def clean(self):
        if self.start_date > self.end_date::
            raise ValidationError("Dates are incorrect")

Something like that will work in the django admin without any need to create a form class.