Answer:
Materials are required to define surfaces, cross-sections, and solids. The material properties affect the stiffnesses of these objects.
There are 13 material models available if you have a license for the RF‑MAT NL add-on module.
In the case of the abundance of material models, it is necessary to make sure that you assign the corresponding material model to the members and their surfaces/solids.
In the example shown here, surfaces have been generated from a member for a detailed analysis. There is still an unused cross-section defined (marked in blue) and the material is entered for the member cross-section as well as for the surfaces. When editing an existing material to Isotropic Nonlinear Elastic 2D/3D, the 2D/3D material model is also defined for the created member cross-section, which leads to the error message.
When working with members and surfaces / solids, we recommend creating more than one material.