my boolean modifier isn't cutting into my shape correctly So I'm trying to use the boolean modifier to use shape A to cut into shape B. But shape A just turns light blue and fails to cut into shape B whenever I apply the modifier.

Whenever I do press apply, the error message "applied modifier was not first, result may not be as expected" pops up. I've tried to move the boolean modifier over the mirror modifier, but that doesn't help.

I've also tried to recalculate normals for both shapes with no success.

does anyone know how to solve this???

Also, here's a copy of my project.
I downloaded your file and the operations worked as follows:


Apply the mirror modifier on object B, Cube in the Outliner  
Make sure only object B is selected, which should be because you just applied the mirror modifier
Add a Boolean modifier and select for the Operation the Difference, see image below with the mark 1
Click on the eye dropper, mark 2  
Select object A, mark 3. The object name Cube.001 in the Outliner.





Hide object A, Cube.001, from the Viewport and from the Render, see image below. The results should be visible without applying the operation but you can apply it at this point if you wish.