Here you are a tutorial to learn how to define in FEMAP a CELAS2 spring element with FEMAP & NX NASTRAN:
In order to enter the three stiffness in the three directions, simply create three elements sharing the same nodes, one per stiffness direction.
But, let me give you a recomendation: forget the spring element, better use the CBUSH element. The generalized spring-damper element CBUSH is a structural scalar element connecting two noncoincident grid points, or two coincident grid points, or one grid point with an associated PBUSH entry.
The advantages of using the CBUSH element over CELASi elements are impressing. For example, if you use CELASi elements and the geometry isn’t aligned properly, internal constraints may be induced. The CBUSH element contains all the features of the CELASi elements plus it avoids the internal constraint problem.
Make sure your RBE2 element don't have repeated nodes under the dependent nodes. What is telling you the error message is that a dependent node is used in more than one rigid element, or it appears more than once in the same rigid element, check your model.