postgresql – ERROR: currval of sequence “rss_sub_source_id_seq” is not yet defined in this session

when I am using this sql to see the current primary key seed of my PostgreSQL 13 table:

select CURRVAL(pg_get_serial_sequence('rss_sub_source', 'id')) 

it tell me that:

SQL Error (55000): ERROR: currval of sequence "rss_sub_source_id_seq" is not yet defined in this session

and this is my id define like this:

ALTER TABLE rss_sub_source 
    ALTER id SET NOT NULL,  -- optional
    ALTER id ADD GENERATED ALWAYS AS IDENTITY 
        (START WITH 1233);

what should I do fix it? how to see the current primary auto incrent seed offset? this is my table DML define:

-- Drop table

-- DROP TABLE public.rss_sub_source;

CREATE TABLE public.rss_sub_source (
    id int8 NOT NULL GENERATED ALWAYS AS IDENTITY,
    sub_url varchar NOT NULL,
    created_time int8 NOT NULL DEFAULT date_part('epoch'::text, now()),
    updated_time bigserial NOT NULL,
    sub_status int2 NOT NULL DEFAULT 1,
    rss_type varchar(64) NOT NULL,
    standard_type varchar(32) NOT NULL DEFAULT 'RSS'::character varying,
    standard_version varchar(32) NOT NULL DEFAULT 'rss20'::character varying,
    cron varchar NOT NULL DEFAULT '0 */1 * * * *'::character varying,
    trigger_count int4 NOT NULL DEFAULT 0,
    next_trigger_time timestamp NULL,
    sub_name varchar(256) NOT NULL,
    last_trigger_time timestamptz NULL,
    tags _int4 NULL,
    source_url varchar(256) NULL,
    sub_type varchar NULL,
    intro varchar NULL,
    remark varchar NULL,
    title_hash varchar NULL,
    failed_count int4 NOT NULL DEFAULT 0,
    lang varchar NULL,
    frequency_month int4 NULL DEFAULT 0,
    reputation int4 NULL DEFAULT 0,
    rep_lastest_refresh_time int8 NULL DEFAULT 0,
    scrapy_take_time int4 NULL DEFAULT 0,
    follower int8 NULL,
    censor_status int4 NULL DEFAULT 1,
    etag varchar(64) NULL,
    last_modified varchar(32) NULL,
    editor_pick int4 NULL DEFAULT 0,
    fav_icon_url varchar(128) NULL,
    dynamic_interval int4 NOT NULL DEFAULT 0,
    local_icon_url varchar(256) NULL,
    CONSTRAINT rss_sub_source_pkey PRIMARY KEY (id),
    CONSTRAINT unique_sub_url UNIQUE (sub_url)
);