Unable to add button in field group using page composer

Summary:

Hi ,

I have a custom object detail layout with fields organised as different field groups.

I am trying to add a button in one of the field groups. Tried using page composer , but it throws error "Add Failed - There are no boxes on the page, so you cannot add any content. Make sure your page (or the template was based upon) includes at least one box before attempting to add content."

Is this feasible ? or the page is not customisable ?

Content (required):

As a user , I should be able to customise custom object layouts.

Version (include the version you are using, if applicable):