When piping system contains FRP piping, the stress summary is not zero at operating case. I would ask experienced FRP engineer if it is code requirement to check operating stress with allowable. Thanks!

Why stress summary reports zero in OPE case when the material is metal but not zero when nonmetal by CAESAR II?

This is not specifically material dependent. Which load cases are checked with the allowable stress depends on the piping code used.
However, as steel and FRP are two different materials with different behaviour, the steel piping codes use different load cases for the stress check in comparison with the FRP piping codes.

Please first study material behaviour and code stress check background before you start doing FRP analysis.