This message is generated when the value of the FaceGrids option does not have the expected form.
If the value of the FaceGrids option is a list, each element in the list should specify a face on which to draw grid lines, and (optionally) the position and style of individual grid lines.
Off[message] switches off the message; On[message] switches it on. For example: Off[FaceGrids::fgstl].