I've been trying to learn to use the media library in Modelica. When I try to use the function setState_pTX and the run the model below it throws an error that says
" Function Modelica.Media.Interfaces.PartialMedium.setState_pTX is neither external nor has an algorithm. It should have been redeclared."
Please tell me how to properly declare the function. Thanks.

model Media
  import Modelica.SIunits.*;
    replaceable package Medium = Modelica.Media.Interfaces.PartialMedium
                         "Medium model" annotation (__Dymola_choicesAllMatching = true);
                        Medium.ThermodynamicState state;
                        parameter Pressure Pair = 1e5;
                        parameter Temperature Tair = 25+273.15;


equation
  state = Modelica.Media.Interfaces.PartialMedium.setState_pTX(Pair, Tair,Medium.reference_X);
end Media;