Some tools don´t work

[Clara5EE7]

hi, i´m new to photoshop and trying to get started. To start i whas thinking that i would edit a picture by darkening the background and lightening the flowers a bit. Now i would like to lighten the bottom flower with the dodge tool but when i klick and/or drag on the picture nothing happens. What am i doing wrong?

[c.pfaffenbichler]

You appear to be working on a white Layer Mask. 

I think you should not apply the Burn and Dodge Tools destructively on an image anyway but work with Adjustment Layers and/or Layer/Blend Mode-combinations. 

[JJMack]

Use something like curves adjustment layer with a luminosity layer mask.