Problems loading pgAudit library on Postgres 12.2
This commit renamed IsSystemNamespace to IsCatalogNamespace in PostgreSQL v12, so you must have built the extension with PostgreSQL v11 or older by mistake.
Check the result of
which pg_config

to see what was first on your PATH when you built the extension.
Be explicit with
make USE_PGXS=1 PG_CONFIG=/path/to/pg12/bin/pg_config