VirtualBox内刚刚安装完Debian9系统，也无法设置共享文件夹。解决的方法就是安装VirtualBox客户端增强包。
1、若直接安装客户端增强包会得到如下提示：
root@debian:/opt# ./VBoxLinuxAdditions.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 5.1.34 Guest Additions for Linux...........
VirtualBox Guest Additions installer
Removing installed version 5.1.34 of VirtualBox Guest Additions...
Copying additional installer modules ...
Installing additional modules ...
vboxadd.sh: Starting the VirtualBox Guest Additions.
Failed to set up service vboxadd, please check the log file
/var/log/VBoxGuestAdditions.log for details.
root@debian:/opt#
不能成功安装
2、备份你的源文件，以免出错后可以恢复。
3、更新Debian9系统安装源文件/etc/apt/sources.list内容如下：
deb http://mirrors.163.com/debian/ stretch main non-free contrib
deb http://mirrors.163.com/debian/ stretch-updates main non-free contrib
deb http://mirrors.163.com/debian/ stretch-backports main non-free contrib
deb-src http://mirrors.163.com/debian/ stretch main non-free contrib
deb-src http://mirrors.163.com/debian/ stretch-updates main non-free contrib
deb-src http://mirrors.163.com/debian/ stretch-backports main non-free contrib
deb http://mirrors.163.com/debian-security/ stretch/updates main non-free contrib
deb-src http://mirrors.163.com/debian-security/ stretch/updates main non-free contrib
4、更新163源之后，更新源缓存
root@debian:~# apt update
5、查看自己的debian的内核版本，命令如下：
root@debian:~# uname -r
6、安装增强包需要的运行环境，执行指令如下：
root@debian:~# apt install build-essential
#安装编译程序必须的软件工具包组build-essential
root@debian:~# apt install linux-headers-`uname -r`
#安装与内核相同版本号的内核头文件
安装完成后，重新启动虚拟机即可安装客户端增强包了。