Hello. Tell me please how to fix it and build a graph? Thank you in advance.

p := PolyhedralSets([-x >= 0, -y >= 0, -z >= 0, -(1/2)*x >= 0, (-x-y)*(1/2) >= 0, (-x-z)*(1/2) >= 0], {y = -20}, [x, y, z]);
plot(p);
Error, (in plot) cannot determine plotting variable
 

Hello. I have the system of inequalities -x / 2> 0, -x> 0, (-x-y) / 2> 0, (-x-z) / 2> 0, -y> 0, -z> 0. Tell me, please, how to build a graph on it?