Symptoms:

   Adding a VM to an existing VPG throws the following error, "A VM cannot be added to the VPG. This is not allowed when you change a datastore."    

Cause:

   This error is most likely due to a VPG Validation failure after updating the VPG.    

Solution:

1. Edit the VPG that user is trying to add the VM
2. Click 'Done' and wait until the VPG update has completed
3. After the VPG update task has completed, add the VM to the VPG and click 'Done'.