When I tried CREATE FUNCTION in PostgreSQL, the following error occurred.

ERROR:  language "plpgsql" does not exist

HINT:  You need to use "createlang" to load the language into the database.

When adding plpgsql later, you can run the createlang command from the command line specifying the database name.

$ createlang -d database_name plpgsql

That’s all from the Gemba.

【Reference】

・PostgreSQL で plpgsql がないときは | dTblog | デザインとプログラムの境界をさまようブログ