BSP_WD_CMPWB : how to add fields from "available fields" to "displayed fields" ?

Hi,

I am trying to add fields that are not displayed in web UI but are only availble in BSP_WD_CMPWB Component Workbench but I have the following error when I try to add them :

Does anyone know what it can be due to and how I can add the fields from the left column to the right one?

Many thanks,

Eli