@denkasyan 

Error, Got internal error in Typesetting:-Parse : "'_Inert_DELAYLESSEQ' is not a valid inert form"

I fixed the * after PolyheadralSet and issued this

@Rouben Rostamian  

Did it work for you? It came out this way:
with (PolyhedralSets);
[AffineHull, Area, CharacteristicCone, ConvexHull, Coordinates,

   Dimension, Display, DualSet, Edges, Equal, ExampleSets, Faces,

   Facets, Graph, ID, InteriorPoint, IsBounded, IsEmpty, IsFace,

   IsInInterior, IsUniversalSet, Length, LinearTransformation,

   LinearitySpace, LocatePoint, Plot, PolyhedralSet, PrintLevel,

   Project, Relations, SplitIntoSimplices, Translate, Vertices,

   VerticesAndRays, Volume, `in`,` intersect`, `subset`]
y: = -20;
                             y: = -20
p: = PolyhedralSet * [- x> = 0, -y> = 0, -z> = 0, - (1/2) * x> = 0, (-xy) * (1/2)> = 0, (-xz) * (1/2)> = 0];
Error, (in unknown) invalid input: lhs received PolyhedralSet: -relations, which is not valid for its 1st argument, expr

@Rouben Rostamian  I need to construct a graph of this system of inequalities with a constant y = -20

@Rouben Rostamian  y = -20 is a constant

@Kitonum Thanks for the help.

@Kitonum PolyhedralSets is always used to construct three-dimensional graphs of the system of inequalities? And you always need to specify non-strict inequalities?

@Kitonum Thank you very much.

@Kitonum I have a task to build a graph on the system that I gave. Initially, I forgot to worry that y is a constant of -20. What you built me is very interesting, but I need a system with strict inequalities. How can i do this?

@Kitonum And if you add the constant y = -20?