This is what you provided, formatted:
This is the content on docker-compose.override.yml file
version: "3.4"
services:
db:
volumes:
- db-volume:/bitnami/postgresql
volumes:
db-volume:
name: db-volume
your list of services needs to be indented under services (same with volumes if you provide them that way):
version: "3.4"
services:
db:
volumes:
- db-volume:/bitnami/postgresql
volumes:
db-volume:
name: db-volume