Prerequisites - Enable My Domain

My Domain must be enabled. To access and confirm this setting, click here

• Confirm that My Domain is present.
• Example: YOURDOMAIN
  (NOTE: During the install process, you will need your domain name, keep it handy!)
• Once My Domain is present and enabled, continue to the next steps by clicking Next Step

Note

If your Domain Name is missing, you must register it. 
Go to Salesforce for more information on how to register your domain.

More information on prerequisites here.