你需要终止 IF 对账单 END IF; . 同时检查变量 NULL ，你需要 IS NULL 而不是 = NULL .

CREATE OR REPLACE PROCEDURE NewUser (g_name VARCHAR, f_name VARCHAR, u_password VARCHAR, u_email VARCHAR, u_dob DATE, p_name VARCHAR, team_id INT)
LANGUAGE plpgsql
AS $$
DECLARE s_user_id INT;
DECLARE p_user_id INT;
BEGIN
    IF p_name IS NULL AND team_id IS NULL THEN 
        s_user_id = (SELECT MAX(user_id) FROM spectator) + 1;
        INSERT INTO spectator (user_id, given_name, family_name, password, email, date_of_birth)
        VALUES (s_user_id, g_name, f_name, u_password, u_email, u_dob);
    ELSE 
        p_user_id = (SELECT MAX(user_id) FROM player) + 1;
        INSERT INTO player (user_id, given_name, family_name, password, email, date_of_birth, player_name, team_id)
        VALUES (p_user_id, g_name, f_name, u_password, u_email, u_dob, u_name, team_id);
    END IF;   --> here
END;
$$;

如果正确缩进代码，这种类型的错误更容易发现。