Delay not work in Running Background Tasks part

Django
1 
@shared_task
def notify_customer(message):
    print('sending email...')
    sleep(2)
    print(message)
    print('email were sent!')


def say_hello(request):
    notify_customer.delay('test')
    return render(request, 'hello.html', {'name': 'mosh'})

it works without delay but nothing print when I set delay

2

Which lesson in Running Background Tasks is this?

3

The Ultimate Django Course ,Running Background Tasks, Creating and Executing Tasks