我对 Linux 很陌生。我在 vmware 上使用 Xubuntu 14.10，我正在尝试制作一个可执行文件。我make在终端中输入，但该过程因错误而停止：
flex -Pfct_pddl lex-fct_pddl.l
make: flex: Command not found
makefile:66: recipe for target 'lex.fct_pddl.c' failed
make: *** [lex.fct_pddl.c] Error 127
据我了解，我需要安装 flex。我试着这样做：
sudo apt-get install flex
但我得到了这个：
Package flex is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'flex' has no installation candidate
知道我应该做什么吗？谢谢。