$ sudo apt-get update
$ sudo apt-get install libxdmcp6-udeb 

libxdmcp6-udeb is installed in your system.

Make ensure the libxdmcp6-udeb package were installed using the commands given below,

$ sudo dpkg-query -l | grep libxdmcp6-udeb *

You will get with libxdmcp6-udeb package name, version, architecture and description in a table

libxdmcp6-udeb package basic information: