This is a simple thing, but because at the moment it’s sort of invisible, you may need a little help understanding it.
The Content Type Hub is a nice capability that lets you create your Site Columns and Content Types in one centralized Site Collection for syndication across your farm. It’s an excellent idea to use it so that your Content Types have consistent definitions and set up across those Site Collections.
In an on premises farm, you’d create a Site Collection to play this role and activate the Content Type Syndication Hub Site Collection Feature.
However, if you try to do this on Office365 in SharePoint Online, you may get an ugly error. Even if you don’t get an error, you may be creating a redundant Content Type Hub.
This is because there is a Content Type Hub Site Collection already provisioned (in the tenants I can see) at /sites/contentTypeHub. You should probably use this Site Collection for your Content Type Hub if it is there.
Unfortunately, this Site Collection isn’t visible in the admin dashboard listing of Site Collections.
To determine if you have this Site Collection, you can go to the /sites/contentTypeHub URL. However, a more conclusive test is to go into Site Settings on one of your existing Site Collections (probably the root one) and click on Content Type Publishing.
On that page, you’ll see a link to the Site Collection that is acting as the Content Type Hub, if there is one: