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.