Welcome back to planDisney, David.
"Oh, bother." I'm sorry you are experiencing this issue with your special event tickets. Now, I need to "Think, think, think." Hmm. "Aha!" Let me try something and I'll be "Backson!"
OK, I'm back, David. I happen to have purchased tickets to Oogie Boogie Bash for my family too. My daughter had linked the tickets to her account already. Using the page to link tickets on the Disneyland Resort website, we attempted to re-link one of the tickets to her account and she got back the message "The ticket or pass entered has already been linked to an account." Does that match the message you received? If so, I think it may mean that your tickets are already linked to the My Disney account you are using for the Disneyland Resort.
Now, if that is the case, where can we find our tickets? This is one of the things that is somewhat similar, but still meaningfully different between the Disneyland Resort and the Walt Disney World Resort. If you are used to using Disney World's My Disney Experience, you might expect to be able to review current Disneyland Resort tickets in a similar way on a web page. However, the Disneyland Resort website doesn't have the same way to verify tickets. You can use the theme park reservations page to make, modify, and view existing park reservations for your regular Disneyland Resort tickets. But special event tickets are date-based and don't require a separate reservation.
This leaves us with one more option for viewing our Oogie Boogie Bash tickets: the Disneyland mobile app. After logging into your Disneyland Resort account in the app, you can open the My Tickets & Passes screen to find not only your regular theme park tickets but also your special event tickets. If it turns out that you still don't see your tickets in the Disneyland mobile app, perhaps it's time to ask Owl for help. Er, I mean, ask a Cast Member for help by either calling the tickets line or sending an email.
I hope that helps solve the mystery of the missing tickets, David. If you should have more questions about planning your visit to the Disneyland Resort during Halloween Time, I hope you'll visit us again soon here at planDisney.
"TTFN! Ta-ta for now," David!
~Tony