Ran into this tonight, wanted to make a quick post for others who may or may not have seen this before:
This is the message I got after right clicking the NFS datastore and selecting unmount. Most commonly, this will happen because Vmware thinks a file from the datastore is still being use in by a VM that is running on this host. Most likely, this is where you kept your ISOs to build your VMs and you forgot to unmount your isos. Edit the properties of your VMs and make sure there are no mounts still pointing to this datastore. The easier way to do this would be to use a PowerCLI script, it’ll take care of things like this for you much faster than the GUI.