Layer not editable

I was following one of the tutorials but at a certain point I got an error message saying:
"Could not use the brush because the content of the layer is not directly editable"
I have no idea what this means. The only thing I did so far was draw a circle and I tried to use the brush.

Did you make the circle with the shape tool? If so, then all you need to do is right click the the layer and then click Rasterize Layer or Rasterize Layer Clipping Path, depending on where you right click. If you made the circle using the selection tool, make sure that none of the circled boxes are checked: . Hope this helps.

Good Luck,