Error in Modelica: Can only call functions that have one algorithm section or external function specification

I wrote a code in ANSYS Twin Builder using Modelica. It throws me an error: "Calling function Medium.density(): can only call functions that have one algorithm section or external function specification " Would you help me why this error arises ? Thanks..

    //Component(s)
    Real f[2*T_per/(T_per/10)]; 
    Real time_interval;
    parameter Real opening_HP;
    Real opening_NP;
    parameter Real N;
    parameter Real T_per;
    parameter Real V_tod;
    parameter Real V_unten;
   
    Modelica.Fluid.Valves.ValveIncompressible NP (
        dp_nominal  = 65.21,
        opening = opening_NP,
        rho_nominal  = 998.388,
        Kv = 0.02178066,
        V_flow(start = 0.0000488573),
        dp = dp(start = 65.211094),
        m_flow(start = 0.048778537));
    Modelica.Fluid.Valves.ValveIncompressible HP (
        Kv = 0.021549519,
        opening=opening_HP,
        dp_nominal  = 66.6175,
        rho_nominal  = 1019.921,
        V_flow(start = 0.0000478258),
        dp(start = 66.6175),
        m_flow(start = 0.048778537));
    Modelica.Fluid.Machines.SweptVolume Swept1 (pistonCrossArea = 0.0001131, clearance = 0.000002, portsData = false);
    Modelica.Fluid.Vessels.OpenTank tank (vessel_ps_static = 400000);
    Modelica.Fluid.Vessels.OpenTank tank1 (vessel_ps_static = 50000000);


equation
    T_per=pi/(180*3600*N/60);//Radian 
    time_interval=T_per/10;
    for i in 1:2*T_per/(T_per/10) loop
      f[i]=((V_unten_V_tod)/2)*sin((2*pi*i*time_interval/T_per)+(V_tod+(V_unten-V_tod)/2));
    end for;
    if der(f)>=0 then
      opening_NP=1;
      opening_HP=0;
    else
      opening_NP=0;
      opening_HP=1;
    end if;
    
    //Connection(s)
    connect(NP.port_a, Swept1.ports[1]);
    connect(HP.port_b, Swept1.ports[2]);
    connect(HP.opening, tank1.ports[1]);
    connect(NP.opening, tank.ports[1]);
end  Hochdruckreiniger2;

Solution

It seems you have not selected medium for Fluid-models, in Modelica you normally do that by modifying as follows:

Modelica.Fluid.Valves.ValveIncompressible NP (
  redeclare package Medium=Modelica.Media.Water.IdealSteam,
  ...

(For NP, HP, tank and tank1.)

Without that the Medium is PartialMedium where Medium.density is a partial function with neither algorithm nor external call.

Note that other tools will detect this in various other ways, and technically calling a function with neither algorithm nor external call is legal since Modelica 3.3 (provided the output variables have declaration assignments - which they don't in this case).

Additionally dp=dp(start=...) should be just dp(start=...), and vessel_ps_static shouldn't be changed like that.