2- Saved it as .msh (V2 ASCII)>convert to .vtu using OGS2MSH
Apart form that I have used the project file form the example and did not changed anything.
There is a difference between the RichardsFlow and the RichardsMechanics process in OGS6. The project file is for a RichardsMechanic process. As far as I know the RichardsMechanics process needs a mesh with higher order elements. You can use the OGS6 tool createQuadraticMesh to generate a mesh with quadratic-order elements.