docker-compose : Unsupported config option for services service: 'web'

I am going through the Getting Started with Docker Compose page.

In Step 3, I made a docker-compose.yml file as described:

version: '2'
services:
    web:
        build: .
        ports: 
            - "5000:5000"
        volumes: 
            - .:/code
        depends_on:
            - redis
    redis:
        image: redis

But when I run:

$ docker-compose up

I get following error:

Unsupported config option for services service: 'web'

What am I doing wrong? I was not able to figure out what is going on.

Support for the version 2 compose file format was introduced in docker-compose version 1.6, released around February of this year.

You're using 1.3.3, from July 2015.

You need to upgrade to a more recent version to use the version 2 format configuration files.

Since this is the first result on Googling "docker-compose Unsupported config option for services", I would like to add that the most common reason as of 2020 is missing of version: "3".

Just add version: "3" to the start of your docker-compose.yml.

From the docs:

There are currently three versions of the Compose file format:

1. Version 1, the legacy format. This is specified by omitting a version key at the root of the YAML.
2. Version 2.x. This is specified with a version: '2' or version: '2.1', etc., entry at the root of the YAML.
3. Version 3.x, the latest and recommended version, designed to be cross-compatible between Compose and the Docker Engine’s swarm mode. This is specified with a version: '3' or version: '3.1', etc., entry at the root of the YAML.