Please make sure of following things and then try again
- Make sure that your currently selected layer is not a background layer. If it is , to remove it choose "Layer>New>Layer From Background".
- Make sure that your image is not in Indexed Color Mode or Bitmap Mode. To change mode choose "Image Menu>Mode" and choose the color mode as RGB or grayscale.
- Also make sure that your image is not in 16 bit mode