I have a problem with my blending mode, I added the Add mode to one of my layers but the background is still there and I don't know what to do to remove it and just keep the second background on my second layer
I am guessing the background of the layer with the add mode is not black. If that is not the problem we need screenshots and details.
Blending modes operate on genuine layers, not the comp background. And contrary to your perception they do not create transparency. you have to explain much better waht you are actualyl trying to do.