_str_dblink = 'dblink(''dbname=' || _dbname || ''',' || '''select ' || _list_field || ' from ' || _table_name || ''')' ;

EXECUTE '
with inup as (
    insert into '  || _table_name || ' (' || _list_field || ')
        select '   || _list_field || '
            from ' || _str_dblink || '
    as t (' || _list_field_type ||   ')
    on conflict (' || _guid || ') do update set ' || _list_field_excl ||
    ' returning id
)
insert into tmp_table select id from inup' ;

ОШИБКА:  функция dblink(unknown, unknown) не существует
СТРОКА 5:             from dblink('dbname=buf_dekanat','select mk_guid...
                           ^
ПОДСКАЗКА:  Функция с данными именем и типами аргументов не найдена. Возможно, вам следует добавить явные приведения типов.
ЗАПРОС:  
with inup as (
    insert into marks (mk_guid,mk_st_guid,mk_sb_guid,mk_at_guid)
        select mk_guid,mk_st_guid,mk_sb_guid,mk_at_guid
            from dblink('dbname=buf_dekanat','select mk_guid,mk_st_guid,mk_sb_guid,mk_at_guid from marks')
    as t (mk_guid character(36),mk_st_guid character varying(36),mk_sb_guid character varying(36),mk_at_guid character varying(36))
    on conflict (mk_guid) do update set mk_guid=EXCLUDED.mk_guid,mk_st_guid=EXCLUDED.mk_st_guid,mk_sb_guid=EXCLUDED.mk_sb_guid,mk_at_guid=EXCLUDED.mk_at_guid
returning id
)
insert into tmp_table select id from inup

dblink('dbname=buf_dekanat', 'select mk_guid,mk_st_guid,mk_sb_guid,mk_at_guid from marks')