First of all, HAVE YOU INSTALLED VirtualBox ??

Next, check that your BOINC installation sets up the correct permissions for running VirtualBox. Recheck our information on "Installing BOINC".

If BOINC is correctly installed, the main reason for that message is that the wrapper fails to remove an old virtual machine (VM) after it has expired, and so cannot register a new VM for the next Work Unit (WU).

The best solution for cleaning your project files if you have to do it manually is:

1. Abort the running WU.
2. Quit BOINC (the core client should be not running).
3. Open VirtualBox Manager, and remove the registered machine named "BOINC_VM" (or any VM's with names "boinc_wu_xxxx_xxx").
4. Open VirtualBox Media Manager and remove the cernvm.vmdk virtual hard disk if it is registered.
5. Launch BOINC again.