How to tell:
      Check the Layers Dialog to
      see if the active layer is actually a
      Layer group.
      When a layer group is not empty, a small icon 
      
        
       or
      
        
       appears in front of the layer group's thumbnail and name.
      Most actions don't work on a layer group, in which case an error message
      will show up: «Cannot paint on layer groups.»
    
How to fix: You need to make a layer active that is not a layer group. Select a layer by clicking it in the Layers Dialog. If the active layer group has a sign in front of it, it is collapsed. You can click it to expand and show the individual layers inside that group.