Chamfer works like this (from the drawing of your 2 lines) Normaly the 2 lines are joined at one angle before a chamfer ;)
So in your case (with lines no joined) select first Horizontal line then Vertical line
If you want only the jonction with your 2 lines (and don't know the measures) use Blend / Tangent(G1) with Bulge = 0
and will obtain also the green line :)