Art history brush not available

Hi 

I'm trying to use the Art history b brush tool. I have imported an image from Lightroom into Photoshop. When I try and use the art history brush, I get several error messages. First one could not rasterise because the pixel data for this layer is not directly editable or secondly, could not rasterise, because the pixel data for this layer is not directly editable. Please can you help me to use the brush to thank you . Ihave tried to change the mode from 16 bit to 8 bit but this has had no effect.