To add a button you must use the Button subsection. This will allow you to format and style the button you wish to use.
Additionally there are a few subsections which already use a button, these are: Hero Image, Attachment, Video and Video Card Horizontal.
Tips!
If you'd like all buttons throughout your content to be uppercase you can set this at the Settings level. Tick on the Settings (gear) icon, select Page, Tick Uppercase Buttons. Any subsection added will now appear in uppercase font.