I've had a chance to look at the actual blend file, and I found geometry problems. Let me show you how I found and fixed one.
I took your Cube cutter into edit mode in a local window. (If you have a number pad on your keyboard the / character on the pad toggles that mode.)
Then I used the select menu to select interior faces
Turns out you've got one:
That's going to confuse the boolean operator, so delete it. Use the X key and select "Faces".
It still doesn't work because you have two objects you need to apply the Boolean to, "Blade" and "Middle Knifu"; so I added the boolean to each.
and it worked.
ASIDE: You have a lot of bad geometry in both the blade and Middle Knifu. In addition to interior faces you have duplicated vertices and you have interior edges.
ORIGINAL ANSWER STARTS HERE:
Be sure that you apply the Boolean modifier to the cube that represents the blade and not the object you are using to 'cut' the hole. Here's a crude example:
What you don't see in the example is the cylinder I used as the 'cutter' object. If you look at my scene collection you'll see why:
When you select the blade and go to the modifiers panel, the Boolean modifier should look like this:
Notice that the modifier is applied to the blade and that the "Object" in the modifier is the cylinder.
Below the Object there's a 'Solver' box. I have it set to Fast.
A common rookie mistake is to apply the modifer but leave the cylinder visible. If you do this it looks like nothing happened:
There are a couple of ways to solve this. For the example I simply made the Cylinder invisible:
If you do it this way, the cylinder will still show up in renders. You have to also make it invisible to the camera:
Alternatively, if you are going to use a lot of Booleans a better technique is to move the cylinder to a new collection, usually called 'cutters':
Select the cylinder and type M. this will bring up a menu:
Since you don't already have a collection, select "new collection" and give the collection a name.
Now all you have to do is uncheck the box for the cutters and the cylinder won't show up in the 3D view or on a render: