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 Domino Administrator or Lotus Domino Designer.
Right now the Domino Administrator is on top! Ensure that the ordinary Notes client is on top and try again.