you are so nicely.
i have tried your suggestion. but it is not work too,
the err information is same,
Other C++ toolchains installed on your system conflict with RTools.
Please run check_cmdstan_toolchain(fix = TRUE) to fix the issue.
i use bush to find my c++ version.
when i put the " c++ --version" it shows that
c++ (Built by Jeroen for the R-project) 8.3.0
Copyright © 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
i am not sure shoud i use more up to date version?
or because of my locale is?:
 LC_COLLATE=Chinese (Simplified)_China.936 LC_CTYPE=Chinese (Simplified)_China.936 LC_MONETARY=Chinese (Simplified)_China.936
 LC_NUMERIC=C LC_TIME=Chinese (Simplified)_China.936
thank you very much.