Why can't I use thicken tool?

[bryce.sims97]

So I am using the tinkercad preferences and I imported a mask I am trying to 3d print, I'm trying to make the sides of it thicker.  I go to the thicken tool and then try clicking on the part I am to thicken but it does nothing?

Here is a picture.  I tried many different things and watched videos but it doesn't even let me select it?

[hamid.sh.]

Thicken is for BRep surfaces, your model is a mesh body. If it's only for 3D printing maybe you can use Mesh > Modify > Shell to add some thickness or hollow out current mesh directly inside your slicer.