Trying to build simple monitor :D
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As Mr Zak says above in comments, face with more than 4 verts often give troubles editing, like here. It is usually better to avoid ngons and stick to "quad" face (4 verts. 3 verts is also bad for animation).
But, if you wish, you could edit your mesh first to make the side face a quad face, using knife tool: in ortho side mode, use K, followed by C - to cut straight lines - and Z to cut through all mesh.
Then click to start the cut,
click to end the cut
and press enter to confirm. You get this.
This allows you to to create a loop (since normal face loop tools won't work either, for the same reason as subdivide):
the side face is now a "quad" and subdivide works: