Thanks for trying the Hortonworks Sandbox.
As this is an issue with the Sandbox it should really have been posted there.
What is going on with your installation of the Sandbox is you need to enable the VT-x/AMD-V Acceleration. You do this by:
- Selecting the Sandbox VM in the list of available VMs in virtual box, do not double click, just single click.
- Open the settings for this VM
- Go to System -> Acceleration
- Make sure that both checkboxes there are checked and select 'OK'
Start the Sandbox VM.
If these are already checked and you still get the error, then either Hardware Virtualization is not enabled or not available on your CPU.