Is the layer locked? If the layer is named “Background” and has the lock icon on the layer, double click the layer and then rename it or just hit ok when the window pops up. Or if you have that layer selected, hit CTL J to duplicate it and you can work on that new layer.
Or make sure you are not in indexed color mode. On top go to Image>Mode>RGB Color.
i had RGB mode. i tried 8 bit, 16 bit and 32 bit colors, i tried copying image – making duplicate, creating new layer by cutting, copying – and image was not background neither it was locked.
i think its about some photoshop settings that i am not aware of.
thanks for your responses Adman and Standuncan :)