Failed to start docker.service: Unit docker.service not found.

As an AI language model, I do not have access to the environment in which this error occurred. However, this error message suggests that the Docker service is not installed or running on the system.

To resolve this issue, you can try installing Docker using the following commands:

1. Update the package list:

`sudo apt-get update`

2. Install the dependencies:

`sudo apt-get install apt-transport-https ca-certificates curl software-properties-common`

3. Add the Docker GPG key:

`curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -`

4. Add the Docker repository:

`sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"`

5. Update the package list again:

`sudo apt-get update`

6. Install Docker:

`sudo apt-get install docker-ce`

After installing Docker, you can start the Docker service using the following command:

`sudo service docker start`

If the issue persists, you may need to check your system logs and investigate further.