How can I fix this? I am getting each time after compiling code in root environment

Newbie
1 
root [0] 
Processing main91.cc...
In file included from input_line_88:1:
/home/pythia8244/examples/main91.cc:15:10: fatal error: 'Pythia8/Pythia.h' file not found
#include "Pythia8/Pythia.h"
         ^~~~~~~~~~~~~~~~~~

ROOT Version: “6.20/04”
Platform: Ubuntu 18.04
Compiler: gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)

2

ROOT Forum -> Search -> main91

3 
feq ($(ROOT_USE),true)
	$(CXX) $< -o $@ -w $(CXX_COMMON) \
	`$(ROOT_BIN)/root-config --cflags` \
	-Wl,-rpath,`$(ROOT_BIN)/root-config --libdir` `$(ROOT_BIN)/root-config --glibs`
else
	@echo "Error: $@ requires ROOT"
endif

As suggested I did this but again showing the same problem

In file included from input_line_88:1:
/home/pythia8244/examples/main91.cc:15:10: fatal error: ‘Pythia8/Pythia.h’ file not found
#include “Pythia8/Pythia.h”
^~~~~~~~~~~~~~~~~~
4

See the Pythia 8 “README” file.
Pythia 8 “examples/main*.cc” are standalone executables. They are NOT any ROOT macros.