I had this same issue have managed to do the migration commands are a bit different as it seems the way Hex OS installs this is not conventional i followed this guild
https://docs.immich.app/install/truenas/#migration-from-old-storage-configuration
unfortunately you need to do this via the truenas interface as Hex OS to my knowledge does not give you access to all that is needed. also worth noting the shell method they tell you to use within truenas scale can time out but you can SSH in if needed this can be turned on in trunasScale interface from Settings System Services SSH then in groups add Truenas_admin this means you dont have to keep moving the mouse while the commands run which was useful as i had over 100GB of data on HDDs so took a while (see image for SSH settings)
Commands used Below ( best to run 1 line at a time so you can see errors) you will need to change HDDs to the path of your application if your pool is not called HDDs
if you need help finding your path it can be seen in the settings for the app from the truenas scale interface (have also put a screenshot) there will be one entry for each of the below sections e.g library , upload , thumbs etc
Click Edit on the Application Info widget.
In the Storage Configuration section
rsync -av /mnt/HDDs/Photos/ /mnt/HDDs/Applications/immich/data/library/
rsync -av /mnt/HDDs/Applications/immich/uploads/ /mnt/HDDs/Applications/immich/data/upload/
rsync -av /mnt/HDDs/Applications/immich/thumbs/ /mnt/HDDs/Applications/immich/data/thumbs/
rsync -av /mnt/HDDs/Applications/immich/profile/ /mnt/HDDs/Applications/immich/data/profile/
rsync -av /mnt/HDDs/Applications/immich/encoded_videos/ /mnt/HDDs/Applications/immich/data/encoded-video/
rsync -av /mnt/.ix-apps/app_mounts/immich/backups/ /mnt/HDDs/Applications/immich/data/backups/
