$ sudo apt-get update
$ sudo apt-get install libx11-6-udeb 

libx11-6-udeb is installed in your system.

Make ensure the libx11-6-udeb package were installed using the commands given below,

$ sudo dpkg-query -l | grep libx11-6-udeb *

You will get with libx11-6-udeb package name, version, architecture and description in a table

libx11-6-udeb package basic information: