Hi Jose, good that you got it working.
Guess values can be a bit tricky. That a variable's start value is used as a guess value indicates that it is solved in a nonlinear system, typically together with other variables. Hence it is typically not only one variable's start value that matters, but several.
This is complicated further by the fact that the initialization is underdetermined, if WSM chooses different values to initialize exactly from their start values this might change which variables and equations are in the nonlinear system. This is one of the reason why we decided to add the "The initialization problem is underdetermined" notification in WSM 5.1.
You should be able to add fixed = true to the variables in the list shown in the "The initialization problem is underdetermined" notification. Your model's initialization should then be exactly determined (with the exception of nonlinear systems that might have more than one solution).
If you would like to fix other variables (whose initial values might be more relevant to change after build) you can try setting fixed = true to those instead (they don't need to be states). WSM will then try to set it up the way you requested, but if it can't you will get some kind of error or warning, like the ones you already saw or a warning that tells you that the initialization problem is overdetermined (which is typically undesirable). For complicated models I would suggest adding fixed = true one variable at a time, checking that you don't get any warnings or errors in the Build Log, then continue until it no longer says the initialization is underdetermined.
