This is a known issue, this is the root cause of the problem.
When you have a datacenter , but no host attached , we get the below mentioned error Error running data acquisition script: You cannot call a method on a null-valued expression.
The workaround for the same is "Removing the empty datacenter"
Request you to open a case with NetApp support against bug 736081.
Here is the public report for the same with workaround.
VMware vCenter data source acquisition fails when the vCenter server includes either no datacenter object or empty datacenter objects
VMware vCenter data source acquisition in OnCommand Workflow Automation (WFA) fails and WFA displays the following error messages:
1. "Error running data acquisition script: Cannot validate argument on parameter 'Datacenter'.
The argument is null or empty. Supply an argument that is not null or empty and then try the command again."
Cause: There is no datacenter object added to a vCenter.
2. "Error running data acquisition script: You cannot call a method on a null-valued expression."
Cause: There are datacenter objects added to the vCenter, but no host is attached to the datacenters.
Remove the empty datacenters or datastores from the vCenter server and retry data source acquisition.