If you are running TrueNAS version 11.3 or greater, you need to create a dataset below the storage pool as you cannot edit ACL permissions for the pool. To do so, follow these directions:
Open the TrueNAS user interface and navigate to Storage –> Pools. Click on the three dots next to the pool and select add dataset.
Give your dataset a friendly name and click on the submit button to create the dataset. Click on the three dots next to the newly created dataset and select edit permissions.
Select the user and group and under the owner fields and then grant them ACLs under the access fields.
When finished, click the save button.