PL/Tcl is not installed by default, if you've used the standard source distribution to install PostgreSQL. If you compiled PostgreSQL from the source, you need to run the configure script with the –-with-tcl option.

If you've used a binary distribution on your platform, you can normally install PL/Tcl using your package manager. You can search for postgresql-pltcl, or a similar package name, as it differs across distributions. Once PostgreSQL is compiled with the correct option, or you have installed the appropriate package, you can create the language using the createlang utility or the CREATE LANGUAGE command:

$ createlang pltcl template1

You can use also use the untrusted version to create the language, as follows:

$ createlang pltclu template1