Hello,
The reason behind the failure could be caused by different issue; start by confirming the below:
1. Verify if you have FlexClone license on the storage end? You can verify by running the below command:
cluster::> license show
2. Verify if the aggregate is part of the agggr-list of the vserver:
cluster::>::> vserver show -vserver <vserver name> -fields aggr-list
As far as the KB you mentioned, those command you would run on the ESX host from a putty session.
Thanks