IF 不是用 sql 语言实现的，而是用 plpgsql 语言实现的，参见 手册 。

您可以将 LANGUAGE sql 替换为 LANGUAGE plpgsql ，然后在函数体中添加 BEGIN 和 END ：

CREATE OR REPLACE FUNCTION public.add_activity(IDactivity smallint,Date_Start date, Data_End date, 
Type character varying,Name character varying DEFAULT NULL::character varying,
Typology character varying DEFAULT NULL::character varying, Client smallint DEFAULT NULL::smallint)
RETURNS void
LANGUAGE plpgsql
AS $BODY$
BEGIN
INSERT INTO public."Activity" 
VALUES(IDactivity, Date_Start, Data_End, Type, Name);
IF Type = 's' THEN
INSERT INTO public."Service" 
VALUES(IDactivity, Typology, Client); 
END IF;
END ;
$BODY$

或者您可以更改代码以保留 sql 语言：

CREATE OR REPLACE FUNCTION public.add_activity(IDactivity smallint,Date_Start date, Data_End date, 
Type character varying,Name character varying DEFAULT NULL::character varying,
Typology character varying DEFAULT NULL::character varying, Client smallint DEFAULT NULL::smallint)
RETURNS void
LANGUAGE sql
AS $BODY$
INSERT INTO public."Activity" 
VALUES(IDactivity, Date_Start, Data_End, Type, Name);