What will be the % error bet ansys and nastran when we choose any structural problems? (condition is same node,geometry,etc..)

In verification of FE analysis one of the  important issue is to show the convergence wrt mesh density. In fact you show that the result is stabilizing /converging/ if you are computing with a set of FE models with refined meshes for the same mechanical model. Typically, the percentage error levels for the case studies  are within  0-20 %.

The lower the error, the more reliable is FE model.