I followed the link below for building an interface for g4-fluka. https://geant4-userdoc.web.cern.ch/Doxygen/examples_doc/html/exFlukaCernCrossSection.html#autotoc_md18
But when I type “make interface”, the shell reports
/geant4-v11.2.1/examples/extended/hadronic/FlukaCern/FlukaInterface/source/fluka5/hadronic_interactions/interface_to_G4/fluka_common_dependencies.hh:39:10: fatal errer：iounit.h: NOT FOUND
I checked the include folder of fluka and there is only the iounit.inc. Most of the xxx.h files seems to be missing.
I am assuming you are using the FLUKA binary packages - in the case of FLUKA source packages, you need to generate the headers with make cpp_headers, as mentioned in the documentation you pointed to (FLUKA paragraph).
There were indeed issues in previously distributed binary packages, in RPM/DEB format only. The .h files were accidentally not included. This was already fixed a few weeks ago in the context of a new automatic packages creation, so will be fine in the next FLUKA release.
In the meantime, you can simply download the FLUKA binaries in tgz format, where the .h are correctly included.