Due to the way TrueNAS is built-duplicating files using Rsync requires some special flags to allow it to work
The below example command has been used and verified as working with TrueNAS Scale
rsync -avrh -A --no-perms <SOURCE> <DESTINATION>
If you want to see progress, add in the flags
rsync -avrh -A --no-perms --progress <SOURCE> <DESTINATION>