Try to use SELECT..INTO clause and simple user variable -
CREATE PROCEDURE ManufactureAdd(
p_manufactureName longtext,
p_address longtext,
p_phone varchar(50),
p_email varchar(50),
p_description longtext)
BEGIN
SELECT IFNULL(MAX(manufactureId + 1), 1) INTO @p_manufactureId FROM tbl_Manufacturer;
INSERT INTO tbl_Manufacturer (manufactureId, manufactureName, address, phone, email, DESCRIPTION)
VALUES (@p_manufactureId, p_manufactureName, p_address, p_phone, p_email, p_description);
SELECT @p_manufactureId;
END