If you launch List Fields via the Actions-menu, you will automatically have the Notes client "on top", and this error will never appear!
But, if you launch List Fields from the operating system, such as via an icon or short-cut, List Fields must have the Notes client "on top of" other Notes clients, such as Lotus Domino Designer or Domino Administrator.
Right now the Lotus Domino Designer is on top! Ensure that the ordinary Notes client is on top and try again.