bucardo sync issue, function does not exist

Question

I have issues with adding database to sync. DB versions: PostgreSQL 9.6.8 (RDS), Jump system: Ubuntu18.04 LTS, bucardo version: 5.4.1.

When trying run this command:

bucardo add sync db_sync relgroup=copying_herd dbs=source_db:source,dest_db:target onetimecopy=1

I receive this error:

WARNING:  Issuing rollback() due to DESTROY without explicit disconnect() of DBD::Pg::db handle dbname=dc_sourcedb;host=xxx.aaa.xyz;port=5432 at line 126.
Failed to add sync: DBD::Pg::st execute failed: ERROR:  DBD::Pg::db do failed: ERROR:  function search_string(text) does not exist
LINE 5:             array_agg(search_string(x)),
                              ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
QUERY:
  SELECT
    coalesce(
        array_to_string(
            array_agg(search_string(x)),
            '|'
        )::text,
        ''
    )
  FROM unnest($1) x

CONTEXT:  SQL function "flatten_text_array" during inlining at line 126. at line 30.
CONTEXT:  PL/Perl function "validate_sync" at /usr/bin/bucardo line 4612.

Function search_string exist and I can run this function from psql command line