Can't Knife when Hidden

Hi Vitor DDamaing ,

You are not doing the Hiding wrong, but you can only use the Knife, where there are Faces.

You might need to Fill the bottom Vertices first, because it looks like there is no Face there, so nothing to cut.

You can always subdivide the edge, no need to use the knife tool exclusively. So select the edge and right click, subdivide and then double tap G to move the vertex where you want it.