Mask in effects will not work

It's not going to work because the image underneath is the same. Use the invert checkbox. Select the image underneath and use the transform function to shift it to the left so there is black under the cut square.

Nothing will happen as long as the top and bottom clip are exactly the same.

As Tom said:

1) make sure that invert mask is checked (still everything looks the same for now)

2) select the *bottom* clip, activate the Transform tool and move it to the left a bit. By doing this the part of the bottom clip that will show through will be a different one, ideally in black to fill the hole cut from the top clip.

Start all over. Delete the clip on top. Make sure there is nothing applied to the clip on the primary storyline, except maybe the color correction.

Option-drag the clip in the storyline straight up to make a duplicate copy attached, one on top of the other.

Apply the Draw Mask to the clip on top, selection the area you want to delete. Nothing will change. Click the invert box. Nothing will change because the clip underneath is identical to the clip that has the mask applied.

Select the clip on the primary storyline and activate the Transform function. Slide the clip to the left so that the content that's seen through the hole on the top clip is different.

a) You have to invert the mask

b) The way you are moving the bottom clip is defeating the intended purpose, which was of using part of its black to "replace" the part cut from the top one. You are moving it complely off the area of the mask so it might as well not be there at all.

Move the bottom clip not down but to the left.

Can you be more specific about what effect you're using and what mask you're using? Perhaps post some screenshots.

It looks like you have the same clips, one on top of the other, and the Draw Mask applied to the top clip. Not sure what you expect to happen.