ERROR! 'sudo' is not a valid attribute for a Play

-  hosts: RemoteMachine1
   name: play2
   become: yes
   tasks: 
      - name: "Find the disk space"
        command: "df -h"
        register: result
      - name: "Install Apache in the remote machine" 
        apt: name=apache2 state=latest
      - debug: var=result.stdout_lines

use become: yes it will run your tasks as the root user.

Become directives

Ansible play keyword sudo was (long ago) deprecated with warnings in version 2.0 and removed in version 2.2

See the actual supported play keywords. Use:

become: true