How to tell: Check the Layers Dialog to see if the active layer is actually a Layer group. If it is, it will have a + or - sign before its name and icon. 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.