Try to avoid index reduction by modifying your balance equations or implement the index reduction manually.

Based on additional thermodynamic equations such as Clausius Clapeyron (https://en.wikipedia.org/wiki/Clausius%E2%80%93Clapeyron_relation), some balance equations might be rearranged so that the index reduction is not necessary. Try to find an analytical/thermodynamic solution to this problem.

Usually, the index reduction is caused by a constraint equation which connects two balance equations. This might happen on system level with two connected volumes as mentioned in the comment by matth. But this might also happen in a volume model if you add a constraint equation on a state variable (such as temperature constraint) or use der() to differentiate some property.

We implemented derivatives for some properties in some medium models in TSMedia/TILMedia. How to define derivatives is described in https://www.claytex.com/blog/applying-derivatives-to-functions-in-dymola/ or https://specification.modelica.org/master/functions.html#derivatives-and-inverses-of-functions. The derivative of the saturated properties of the mentioned mediums is not implemented for the equation of state based models in our library, so you will get the same error. You try the freely available library from https://github.com/TLK-Thermo/TILMediaClaRa, but the desired mediums are not included. Using spline interpolation fluid property models, the derivative is implemented in TSMedia/TILMedia. We avoided index reduction when implementing the TIL Library.