$ sudo apt-get update
$ sudo apt-get install libtcl8.6 

libtcl8.6 is installed in your system.

Make ensure the libtcl8.6 package were installed using the commands given below,

$ sudo dpkg-query -l | grep libtcl8.6 *

You will get with libtcl8.6 package name, version, architecture and description in a table

libtcl8.6 package basic information: