trouble using brush tool with PNG files in Photoshop

Hi guys, new to photoshop, im trying to use the black brush tool within a mask to erase part of an image, however when i activate the mask and activate the brush tool with 100percent opacity and hardness, nothing happens. When i try to use the brush tool on the orginal image layer, again nothing also happens. Is there some setting i need to change? do i need to change the file of the image?

ive atached a screen shot of what it looks like when i try using the brush tool on the image