This method or property must be called within the lambda passed to QueuedTask.Run.
Remarks
Symbol Layer Drawing, SLD, cannot be set/added for layers with unclassed symbols, proportional symbols, dictionary symbology, or for polygon feature layers with graduated symbols. CanAddSymbolLayerDrawing() will also return false if the input layer already has SLD added (unless the SLD is invalid) or is a child of a group layer that has SLD added. When SLD has been established for a group layer, it cannot be set for individual layers within the group; all SLD control is set through the parent group layer. SLD is not available for 3D layers. It can only be set on feature layers in a map or in the 2D Layers category of a scene.
QueuedTask.Run(() =>
{
//check if it can be added to the layer
if (featLayer.CanAddSymbolLayerDrawing())
featLayer.AddSymbolLayerDrawing();
//ditto for a group layer...must have at least
//one child feature layer that can participate
if (groupLayer.CanAddSymbolLayerDrawing())
groupLayer.AddSymbolLayerDrawing();
});