Are you connected to a vCenter or an ESXi node?
If that datastore is managed by a vCenter, you can't do the rename when connected to the ESXi node.
You have to be connected to the vCenter
I just tested that, and I see the same behavior you are seeing.
The Set-Datastore cmdlet might return some more info on the why.
From the ESXi Host Client you'll notice that the Rename Action is greyed out.