Just in case anyone reads this post looking for a solution:
Ignore the validation error and the DDX should run as advertised in the documentation.
My expertise is Java development.
After a lot of trial and error, I now avoid workbench processes completely and use the Java API (AssemblerServiceClient, ReaderExtensionsServiceClient, etc).
This allows much easier runtime generated DDX, exception handling, transaction handling, deployment and maintenance.
Too bad you still need a Livecycle server to execute these atomic PDF operations.
So you want to add x number of attachments to a documents using assembler.
This can be done by using the DDX. Can you please send me your process lca to
I will fix your problems and send it back to you