Help with configuring pgAudit
Before you can use pgAudit, you need to add it on the postgresql.conf file. You need to uncomment the line (or add pgaudit if you already have another libraries) to the shared_preload_libraries, like this:
shared_preload_libraries='pgaudit'

After this you must start the server and add the pgaudit extension:
CREATE EXTENSION pgaudit

From then on you can configure what is going to be logged either directly on the postgresql.conf file or by set commands. pgAudit can register by session or by oject created, that depends on you.
Only after all of that your set pgaudit.log = 'ddl' will register what you want.