Motion tile not working (please help)

Motion tile uses the entire layer size. If there is a mask or an alpha channel those areas will be included and, as shown in the screenshot above, you'll get black bars. These look black:

But they are actually transparent:

If you need to keep the mask pre-compose moving all attributes to the new comp then resize the comp to match the mask or use the Region of Interest and crop the comp to the region of interest. You could also scale up the layer with the mask until the mask is the same size as the comp panel.

The other fix, delete the mask. Motion Tile will always render any transparent pixels in the layer and if that transparency is on the edges you'll get black lines between the tiles.