when compiling error "cc1plus: fatal error: gengtype-lex.cc: No such file or dir"

[Gwen Fu via Gcc-help <gcc-help@xxxxxxxxxxx>]

OS: Ubuntu 22.04  X86_64
gcc version :11.04

After having compiled for almost 10 minutes , the error occurred
here is the description:
/bin/bash ../../gcc/gcc/../move-if-change tmp-version.h version.h
echo timestamp > s-version
g++ -std=c++14 -c   -g -DIN_GCC    -fno-exceptions -fno-rtti
-fasynchronous-unwind-tables -W -Wall -Wno-error=narrowing -Wwrite-strings
-Wcast-qual -Wno-format -Wmissing-format-attribute
-Wconditionally-supported -Woverloaded-virtual -pedantic -Wno-long-long
-Wno-variadic-macros -Wno-overlength-strings  -DHAVE_CONFIG_H
 -DGENERATOR_FILE -I. -Ibuild -I../../gcc/gcc -I../../gcc/gcc/build
-I../../gcc/gcc/../include  -I../../gcc/gcc/../libcpp/include  \
        -o build/gengtype.o ../../gcc/gcc/gengtype.cc
/home/gwen/github_repo/gcc/missing flex  -ogengtype-lex.cc
../../gcc/gcc/gengtype-lex.l && { \
  echo '#ifdef HOST_GENERATOR_FILE' > gengtype-lex.cc.tmp; \
  echo '#include "config.h"'       >> gengtype-lex.cc.tmp; \
  echo '#else'                     >> gengtype-lex.cc.tmp; \
  echo '#include "bconfig.h"'      >> gengtype-lex.cc.tmp; \
  echo '#endif'                    >> gengtype-lex.cc.tmp; \
  cat gengtype-lex.cc >> gengtype-lex.cc.tmp; \
  mv gengtype-lex.cc.tmp gengtype-lex.cc; \
}
/home/gwen/github_repo/gcc/missing: 81: flex: not found
WARNING: 'flex' is missing on your system.
         You should only need it if you modified a '.l' file.
         You may want to install the Fast Lexical Analyzer package:
         <http://flex.sourceforge.net/>
make[3]: [Makefile:3408：gengtype-lex.cc] 错误 127 (已忽略）
g++ -std=c++14 -c   -g -DIN_GCC    -fno-exceptions -fno-rtti
-fasynchronous-unwind-tables -W -Wall -Wno-error=narrowing -Wwrite-strings
-Wcast-qual -Wno-format -Wmissing-format-attribute
-Wconditionally-supported -Woverloaded-virtual -pedantic -Wno-long-long
-Wno-variadic-macros -Wno-overlength-strings -Wno-error -DHAVE_CONFIG_H
 -DGENERATOR_FILE -I. -Ibuild -I../../gcc/gcc -I../../gcc/gcc/build
-I../../gcc/gcc/../include  -I../../gcc/gcc/../libcpp/include  \
        -o build/gengtype-lex.o gengtype-lex.cc
cc1plus: fatal error: gengtype-lex.cc: 没有那个文件或目录
compilation terminated.
make[3]: *** [Makefile:3185：build/gengtype-lex.o] 错误 1
make[3]: leave dir“/home/gwen/github_repo/objdir/gcc”
make[2]: *** [Makefile:5055：all-stage1-gcc] 错误 2
make[2]: leave dir“/home/gwen/github_repo/objdir”
make[1]: *** [Makefile:26356：stage1-bubble] 错误 2
make[1]: leave dir“/home/gwen/github_repo/objdir”
make: *** [Makefile:1100：all] 错误 2