When federating Office 365, it is not possible to use the domain that is set as default in Office 365. Should you wish to federate the default domain you will need to change the default to another domain. The onmicrosoft.com domain is recommended for this.
Go to https://portal.office365.com and log in with the your global admin credentials.
Go into the admin centre.
Navigate to Setup > Domains and select one of the domains that you do not wish to federate (onmicrosoft.com recommended). Set that to Default.
We will now be able to federate the provided domain(s).