Try running "sudo a2ensite openproject."
If it installs correctly, it should work.

Running on a Raspberry 4b 4gig, with Ubuntu 23.4 / 64bit installed:

sudo apt-get update -y
sudo apt-get full-upgrade -y
curl -sSL https://get.docker.com | sh
sudo usermod -aG docker $(whoami)
sudo docker pull portainer/portainer-ce:linux-arm
sudo docker run --restart always -d -p 9000:9000 -v /var/run/docker.sock:/var/run/docker.sock -v portainer_data:/data portainer/portainer-ce:linux-arm

These steps will set up docker and compose, then run portainer. From there, I managed to get openproject started with the following stack defintion:

version: "3.7"

networks:
  frontend:
  backend:

volumes:
  pgdata:
  opdata:

x-op-restart-policy: &restart_policy
  restart: unless-stopped
x-op-image: &image
  image: openproject/community:${TAG:-12}
x-op-app: &app
  <<: [*image, *restart_policy]
  environment:
    OPENPROJECT_HTTPS: "${OPENPROJECT_HTTPS:-true}"
    OPENPROJECT_HOST__NAME: "${OPENPROJECT_HOST__NAME:-localhost:8080}"
    OPENPROJECT_HSTS: "${OPENPROJECT_HSTS:-true}"
    RAILS_CACHE_STORE: "memcache"
    OPENPROJECT_CACHE__MEMCACHE__SERVER: "cache:11211"
    OPENPROJECT_RAILS__RELATIVE__URL__ROOT: "${OPENPROJECT_RAILS__RELATIVE__URL__ROOT:-}"
    DATABASE_URL: "${DATABASE_URL:-postgres://postgres:p4ssw0rd@db/openproject?pool=20&encoding=unicode&reconnect=true}"
    RAILS_MIN_THREADS: ${RAILS_MIN_THREADS:-4}
    RAILS_MAX_THREADS: ${RAILS_MAX_THREADS:-16}
    # set to true to enable the email receiving feature. See ./docker/cron for more options
    IMAP_ENABLED: "${IMAP_ENABLED:-false}"
  volumes:
    - "${OPDATA:-opdata}:/var/openproject/assets"

services:
  db:
    image: postgres:13
    <<: *restart_policy
    stop_grace_period: "3s"
    volumes:
      - "${PGDATA:-pgdata}:/var/lib/postgresql/data"
    environment:
      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-p4ssw0rd}
      POSTGRES_DB: openproject
    networks:
      - backend

  cache:
    image: memcached
    <<: *restart_policy
    networks:
      - backend

  proxy:
    <<: [*image, *restart_policy]
    command: "./docker/prod/proxy"
    ports:
      - "${PORT:-8080}:80"
    environment:
      APP_HOST: web
      OPENPROJECT_RAILS__RELATIVE__URL__ROOT: "${OPENPROJECT_RAILS__RELATIVE__URL__ROOT:-}"
    depends_on:
      - web
    networks:
      - frontend

  web:
    <<: *app
    command: "./docker/prod/web"
    networks:
      - frontend
      - backend
    depends_on:
      - db
      - cache
      - seeder
    labels:
      - autoheal=true
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8080${OPENPROJECT_RAILS__RELATIVE__URL__ROOT:-}/health_checks/default"]
      interval: 10s
      timeout: 3s
      retries: 3
      start_period: 30s

  autoheal:
    image: willfarrell/autoheal:1.2.0
    volumes:
      - "/var/run/docker.sock:/var/run/docker.sock"
    environment:
      AUTOHEAL_CONTAINER_LABEL: autoheal
      AUTOHEAL_START_PERIOD: 600
      AUTOHEAL_INTERVAL: 30

  worker:
    <<: *app
    command: "./docker/prod/worker"
    networks:
      - backend
    depends_on:
      - db
      - cache
      - seeder

  cron:
    <<: *app
    command: "./docker/prod/cron"
    networks:
      - backend
    depends_on:
      - db
      - cache
      - seeder

  seeder:
    <<: *app
    command: "./docker/prod/seeder"
    restart: on-failure
    networks:
      - backend

I am not sure from where I copied the original file; but on https://hub.docker.com/r/openproject/community/tags you can see that v12 is only a few days old. In an earlier attempt, I had to still grab a dev release.