Hi @Libeiyu,
I would say that, for Pythia6 support to be enabled in ROOT, you have to specify -Dpythia6=ON on your CMake command line and make sure that the ROOT build configuration is able to find the installation path of Pythia headers / libraries. Could you please post here CMake’s output?
Also see this related post: How to install ROOT with Pythia6.
Cheers,
J.