A common error is that the map used and the layer to displace are not the same size,
The map/layer used for displacement and the layer to be displaced should be at the same size and be referenced at the top left corner, thus > Layer to image size for both will do it
The map/layer used for displacement and the layer to be displaced should be at the same size and be referenced at the top left corner, thus > Layer to image size for both will do it
Patrice