You have to [`Apply Scale`][1] to your mesh first in *Object Mode*.

<kbd>Ctrl</kbd>+<kbd>A</kbd> > *Scale*.

Now try <kbd>I</kbd> to [`Inset Faces`][2] again.


  [1]: https://docs.blender.org/manual/en/latest/scene_layout/object/editing/apply.html
  [2]: https://docs.blender.org/manual/en/latest/modeling/meshes/editing/face/inset_faces.html