In your first screen shot, there's just the one layer. In your second screen shot, that layer is over a solid. You can't use a blend mode with just one layer -- it't cant react to the layer below it.
yes i know but i used to be able to do it a few minutes before. When I would import a stock footage to my comp i would immediately click modes then screen, and it would just be transparent with the checkered pattern in the background but not it shows black.
You are mistaken. The screen mode does not create transparency it just adds pixel values that have a higher luminance value than black. Screen never produces transparency, never did, never will.
You could use Knoll Unmult to create transparency from the black areas in an image. That's the easiest work around.
What Rick said. The perception of things going transparent with certain blending modes is merely incidental. It's not actual transparency.