I received the following error message during the update:
Stopping PostgreSQL 15 database server:
main.Stopping PostgreSQL 18 database server:
mainError: The cluster is owned by group id 993 which does not exist ... failed!
failed!
The trick to fix this was to remove the old upgrade remnants and correct the permissions:
rm -rf /var/discourse/shared/standalone/postgres_data_new
chown -R 999:999 /var/discourse/shared/standalone/postgres_data
… Then run ./launcher rebuild app twice, and optionally (if the migration succeeded and everything is running) ./launcher cleanup.
Hope this helps. 