I think you are confusing "variables" and "parameters" here. I suggest to check any Modelica book which explains the difference between those two types of "variables". I will recommend to check the book of Michael Tiller as well https://mbe.modelica.university/behavior/equations/variables/.

In short a parameter is a "variable" which is supposed to be known a prio-ri. It is similar to a constant and can't hence not be changed in an part of the equation section. This is the reason why the compiler is saying that your system is overdetermined.

You should remove the parameter attribute in front of parameter SI.MassFlowRate errorInFlow

to turn errorInFlow into a variable and make your system work.work.