Exception when calling Postgresql function returning a local numeric from C#
It seems like you misunderstood the RETURNING clause. It should list the columns the statement should return, not the names of the variables the returned values should go into. You need an additional INTO clause for the latter.
CREATE
 OR REPLACE FUNCTION public.func_insert_item(_name character varying)
                     RETURNS bigint
                     LANGUAGE plpgsql
AS
$$
DECLARE
    _item_id bigint;
BEGIN
    INSERT INTO public.items
                (name)
                VALUES (_name)
                RETURNING &lt;name of the id column&gt;
                          INTO _item_id;
    
    RETURN _item_id;
END;
$$

Unfortunately you didn't provide the table's DDL, so I don't know what column could be the ID you want to return. Replace &lt;name of the id column&gt; with its name.