I assume you are connected to the vCenter called Vcenter-Lab ?
If not, first do a connect with
Connect-VIServer-ServerVcenter-Lab
If that is the only vSphere server you are connected to, then you can do
New-Datacenter-nameTest
If you are connected to multiple vSphere servers, you can use the Server parameter like this
$vc=Connect-VIServer-ServerVcenter-Lab
New-Datacenter-nameTest-Server$vc
That is what I mean with the Server parameter.