Docker Error: 'Couldn't connect to Docker daemon'

Introduction

Problem description

The following error is shown when running "docker-compose up" or "docker-compose down":

ERROR: Couldn't connect to Docker daemon at http+docker://localhost - is it running?

If it's at a non-standard location, specify the URL with the DOCKER_HOST environment variable.

Troubleshooitng Procedure

Run the same command with ‘Sudo’ : sudo docker-compose down