I own the digital copy of the 2014 DMG which should provide access to the Oathbreaker subclass for Paladin.

I have made this Paladin level 7 but am unable to select Oathbreaker as a subclass.

Is there something I'm doing incorrectly here?

 https://imgur.com/a/ted1OiN

I saw a note on Reddit about signing out and back into DnD Beyond to potentially resolve the issue and this did it for me. Unsure why it happened but if anyone else has this struggle hopefully it'll be helpful.

Try this: 

Go to your account -> go to licenses -> press sync.  

Bought content should now be synced with your account for use. 

This fixed it for me!  Thank you!