Shape stays the same and only add cuts

[Worlrl]

hi.
i,m doing lesson 12 in section 1 “simple houses”
and when im doing what hes doing it give me this :

the cube keeping the shape instead of making the “roof”
what can cause this?

thanks in advance

[NP5]

You probably have two cubes sitting right on top of each other.

In fact we can see you have.
The ‘hashing’ diagonal shadows on the lower front rectangular face. That is Blender telling you two bits of geometry are fighting to be displayed in the exact same place.

Just start again delete that object. So simple it is easier than fixing it by selecting just one of the cube and deleting that.