vim cannot fold cpp and golang code

[YZ Xie]

vim cannot fold test.cpp, vim prompts E490 no fold found, but rename this file to test.c or test.cc, vim can use zc to fold this same code.

I have set configure in .vimrc foldmethod=syntax set foldlevel=0 set foldnestmax=2 VIMRUNTIME=/usr/share/vim/vim80

in /usr/share/vim/vim80/ftplugin, there exits a cpp.vim file. 

why vim cannot fold *.cpp and *.go, any problem?