ModuleNotFoundError: No module named 'slackclient'

Issue

ModuleNotFoundError: No module named 'slackclient' when using:

from airflow.operators.slack_operator import SlackAPIPostOperator

Fix

Slack changed the name of their import.

To fix, add slackclient>=1.0.0,<2.0.0 to your requirements.txt file, which will be automatically generated via the Astronomer CLI when you initialize your Airflow project on Astronomer.