Erik Kostson
Ansys Employee

The reason might be as per the Ansys help manual and under the strain energy section, that it (strain energy) includes plastic strain which we can not have in a harmonic analysis since it is a linear analysis (so only linear material, hence no plasticity can be included).

One can add perhaps some user defined results – see worksheet summary under the solution in the mechanical tree and there should be some energy results (e.g., ENERGYPOTENTIAL).