A final parameter can't be modified. Because `RealInput YY` is of higher variability (it is allowed to change at simulation time) the value of `R_ic` would need to change if `YY` would change and that contradicts the final keyword.

Make `Component R_ic` and `YY` parameters and let the compiler handle it. You can still change the parameter `YY` from outside.
If variable `YY` is only needed at initialication the compiler will figure that out,use it only at the initialization system and remove it from the simulation system.

  parameter Real X1 = 200;

  parameter Real R_ic = YY * 1 / X1;

  parameter Real YY;

  Modelica.Blocks.Continuous.FirstOrder firstOrder(T = 5, k = 1, y_start = R_ic);

Make `Component R_ic` and `YY` parameters and let the compiler handle it. You can still change the parameter `YY` from outside.

Code:

  parameter Real X1 = 200;

  parameter Real R_ic = YY * 1 / X1;

  parameter Real YY;

  Modelica.Blocks.Continuous.FirstOrder firstOrder(T = 5, k = 1, y_start = R_ic);

Thank you!
How can I still change the parameter `YY` from outside as it is no longer input? I am setting `YY` an input because my model here will be as a black box, in which `YY` will be connected to a previous block in the Diagram view. Eventually, I want to use this black block as FMU in another simulation tool.

If you use a "normal" FMU you can change parameters with some fmi2SetXXX function. I'm not sure what is changeable in a black-box fmu, I guess basically nothing besides inputs.

I think it is possible for parameters to be inputs as well. At least OpenModelica doesn't issue a warning about it.

Maybe that works for you:

  parameter Real X1 = 200;

  parameter input Real R_ic = YY * 1 / X1;

  parameter Real YY;

  Modelica.Blocks.Continuous.FirstOrder firstOrder(T = 5, k = 1, y_start = R_ic);