So in 2019 in a MSUP Harmonic Response, there was some code to warn you that if Constant Structural Damping was the only damping then the warning message was "system is solved without damping".
Then in 2022, they support Constant Structural Damping. Is it possible that they forgot to delete the code that issues the warning message? Maybe.
I like Erik's suggestion, solve once with 0.0 Structural Damping and solve again with a 0.1 Structural Damping and see if the magnitude of the peak response is reduced.