Unable to Make 2014 Rogue Assassin

I'm trying to make an Assassin with the 2014 PH, and in the character creation tab, I have the "2014 Core Rules" box checked.  But when making my character, Assassin does not show up.  And when I uncheck "Core Rules" I lose the only subclass I had, Thief.

Piggybacking this post as opposed to starting a new one. I have tried all variations but can't get the Assassin subclass at third level. 

I've tried 2024 and 2014 checked, 2024 only and 2014 only - but it won't show. Any further advice would be gratefully received

That's the problem I overlooked. I didn't realise I needed to own the DnD Beyond Player's Handbook in order to use the subclass. 

Many thanks for helping me out - much appreciated