PostgreSQL function gen_random_uuid() not working

You need to load the pgcrypto extension in the current database/schema with

CREATE EXTENSION pgcrypto;

like this (tested with PostgreSQL 12):

# SELECT gen_random_uuid();
ERROR:  function gen_random_uuid() does not exist
LINE 1: select gen_random_uuid();
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
# CREATE EXTENSION pgcrypto;
CREATE EXTENSION
# SELECT gen_random_uuid();
           gen_random_uuid            
--------------------------------------
 19a12b49-a57a-4f1e-8e66-152be08e6165
(1 row)