That's all I'm looking for, an answer to the question in the Thread Title: Why doesn't The Undying show up as a Patron option?

Are any of the following true for you?

1. You own the book it was published in (Sword Coast Adventurer's Guide)
2. You have purchased the individual subclass option rather than the full book
3. You are in a campaign where
   • Content sharing is enabled, and
   • Someone in the campaign satisfies either 1 or 2

If none of these are true, you won't have access to the Undying patron

I'm almost certain that criterion (3) is satisfied: our DM has shared SCAG with those of us in the campaign, which I assume they could not do if they did not own the sourcebook.

Is your character specifically in that specific campaign? If not, that’s your problem. The content isn’t being shared with “you,” it’s being shared with “that campaign.” If it is, then you and the DM should both follow this link and make sure your Account Entitlements are in sync: (https://www.dndbeyond.com/account/marketplace). If that doesn’t do it, then the DM should turn Content Sharing off and back on again.

That appears to be the case, as I am trying to build a character that isn't in that campaign. Checking the character that is in the campaign, The Undying is an option. 

Thanks for the assistance!