On TrueNAS SCALE 20.10, when adding an NFS share for the first time, the UI helpfully shows a prompt to enable and start the NFS service. This prompt is not shown again, even if you remove and recreate an NFS share.
If you remove all NFS shares after starting the NFS service, the service automatically stops as seen in System Settings -> Services. However, re-adding an NFS share does not show the prompt mentioned above or restart the NFS service. So you have to manually start NFS from System Settings -> Services for the newly-created share to be exported.
Create an NFS share for the first time. Enable the NFS service through the prompt that appears after creating the share. Check System Settings -> Services to see that the NFS service is enabled. Delete the NFS share you created, then wait about a minute to check System Settings -> Services. NFS has stopped. Create a new NFS share, then check System Settings -> Services again. No prompt has appeared to restart the NFS service, and it has not been restarted automatically. NFS service is still in the stopped state and needs to be manually restarted from System Settings -> Services.
When an NFS share is created, the NFS service should automatically be restarted to ensure the share is usable without any manual intervention.