Valentin wrote

I'm using the Version 1.5. After reviewing the tutorial from:

https://www.featool.com/doc/index.html im a little confused, that in my version after starting the tool and selecting 2D, there is no possibility to select the navier-stokes-equation. What's the reason for that.

The CFDTool toolbox is based on the FEATool Multiphysics simulation platform and features a simplified interface for fluid flow and heat transfer essentially hiding the equations as much as possible. This means that the Navier-Stokes equations will automatically be used if you select an incompressible flow problem (so there is no need for you to explicitly select it). If you wish to have more control of the problem definition (see and select equations, more multiphysics, equation editing and custom PDE equations, as well as scripting in Matlab) then you will have to use the FEATool toolbox instead of CFDTool (even though the equations solved are the same).

The tutorial instructions when selecting a tutorial in the GUI from the "File > Models and Tutorials" menu will show the exact steps to take to set up and solve a model irrespective if you are using the CFDTool or FEATool toolbox (even if the tutorial models are the same they are adapted to correspond to the interface available with the toolbox, while the on-line documentation section currently show the steps for using FEATool Multiphysics).

Valentin wrote

Afterwards im trying to simulate a structure with a compressible fluid but get the error message "Warning: Matrix is singular to working precision". Where does this error come from?

This error typically arises due to a mathematically ill-posed problem formulation, which can be due to incompatible or insufficient boundary conditions, material coefficients, or an otherwise mathematically incorrect PDE formulation. It is impossible to say exactly what as it is model specific.