You cannot extrude the following: Polylines that have crossing or self-intersecting segments. Objects contained within a block.

Which object Cannot be extruded in AutoCAD?

2D polylines with crossing segments cannot be extruded. Thickness and width are ignored. The extrusion extends from the center line.

Can you extrude a surface?

You can select either a surface or the face of a solid body. You can also preselect the 3D faces to extrude before clicking Insert > Surface > Extrude. Select other faces to define the extrude as required. You can select faces that are not touching when you create a surface extrude in a given direction.