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."
This error is most likely due to a VPG Validation failure after updating the VPG.
- Edit the VPG that user is trying to add the VM
- Click 'Done' and wait until the VPG update has completed
- After the VPG update task has completed, add the VM to the VPG and click 'Done'.