Hi Cansu,
This message means that your rate rule could not be evaluated because it is invalid for some reason. This could happen if you did not correctly define one of the parameters you reference in the rule. Look for red indicator marks next to your rules. If you hover your mouse cursor over an indicator, it might tell you which parameters are not defined.
Also, if a parameter is used in a rate rule, it must be associated with a model and not with a reaction. Parameters that are associated with reactions can only be used in reaction rates. This allows different reactions to use the same names for rate constants, even if they have different values. For example, reaction 1 might have a parameter k=1.5, while reaction 2 might have a parameter k=3.
However, I see a larger problem with your model. It looks like you are trying to define your model using both reactions and rate rules. If you use reactions correctly, you do not need to write these rate rules. SimBiology will automatically determine the differential equations from all the reactions in your model.
I recommend you learn a little more about how to use SimBiology before trying to build your model. Take a look at this video tutorial that shows how to build a model in SimBiology. And here is a longer video that introduces model building with the SimBiology Desktop, along with other concepts relevant to pharmaceutical applications.
Good luck!
Arthur
