This is the recommended non-destructive method to give thickness to a flat plane. : Click on your plane in Object Mode.

: Press E to activate the Extrude tool. Move your mouse or type a numerical value to define the height of the post.

: Go to the Modifier Properties tab (wrench icon) and select Solidify from the "Generate" column.

: Check Even Thickness for uniform results. You can later "Apply" the modifier if you need to bake the geometry. 2. Manual Extrusion

If you already have the "walls" of a post but it is hollow (missing top/bottom caps):