Verify the paths are correct. You may really have mounts that are identified by UUID instead of by label.
Make sure the user the container runs as has write permission to those paths.
version: "2"
services:
transmission:
image: linuxserver/transmission
container_name: transmission
environment:
- PUID=998
- PGID=100
- TZ=America/Toronto
- USER=admin #optional
- PASS=****** #optional
volumes:
- /srv/dev-disk-by-label-files/config/transmission:/config
- /srv/dev-disk-by-label-files/Torrents/downloads:/downloads
- /srv/dev-disk-by-label-files/Torrents/watch:/watch
ports:
- 9091:9091
- 51413:51413
- 51413:51413/udp
restart: unless-stopped