The screenshot indicates there are other active snapshot tasks attached to the VM. Based on the error message, assuming the error popped up because the volume is busy being read by another snapshot process (as opposed to the error popping up because something went horribly wrong) it sounds like the solution would of been for you to wait and try again later.. which if you did, everything should of worked. If you're still having problems, you can list pending tasks and cancel them with xe task-list and xe task-cancel uuid=. Use at your own risk.