This a limitation of Nastran nonlinear materials. See the note below from the NX Nastran Basic Nonlinear Handbook:
In NX Nastran, you can perform nonlinear elastic material analyses using isotropic materials. To
define the material properties for a nonlinear elastic analysis, use the MAT1 bulk data entry in
conjunction with the MATS1 and TABLES1 entries to define the stress-dependent material properties.
On the MATS1 entry, when you’re creating a nonlinear elastic material, use field 4 to set the TYPE
option to NLELAST.
Thank you for answering my question so quickly. I am aware of the capability of nonlinear elastic properties for isotropic materials but I was looking for the same in orthotropic materials. I suppose an approximation with an isotropic material will have to do. Thanks again for the reply.