Material region selection in multiphase analysis

I want to simulate Rayleigh-Taylor instability between two fluids of different densities. this is the project. Please help me with the problem because I’m completely new to the platform.
this is the project:

Hi there!

The multiphase fluid flow analysis is used to simulate the behavior of two fluid phases and their interactions. The analysis is carried out using the Volume of Fluid (VoF) method, which is a standard approach for the computation of multiphase systems.

Have a look at this guide for multiphase projects: Multiphase Fluid Flow Analysis | Analysis Types | SimScale

And if you wish to learn more in-depth details regarding the solver, make sure to read this blog:Multiphase Flow in CFD: Basics and Modeling | SimScale Blog

Best regards,
Fillia

I tried following the instructions and guidelines, but the problem was not solved. I even tried to create a whole new simulation from scratch. that also did not solve the problem. Please help me with this, because the simulation is very basic in nature, and failure to accomplish this one is making me uninterested in the platform.
I think the problem is with meshing, but I’m not being able to solve it anyway.
New model: