Modeling hyperelastic material using "neo hookean"

I'm trying to model a hyper elastic material using the "Neo- hookean" proporties. It is my first time working with hyper elastic materials, and the solving of the problem is causing me trouble.

The model with the boundary conditions is attached. When trying to solve I do not reach convergence due to a too small number of iterations, however when iterations are increased it is still not able to solve. the error says "Attempt to evaluate real square root of negative number."

As for the Neo-Hookean material, I'm not familiar with the lambda value, but I have tried to use several different options with no luck. Can this be my problem?

I have tried changing the mesh, the boundary conditions and the material conditions, but I still can get it to solve. Can anybody give me a hint on what to do?

Thanks