Please Help !! I am getting ERROR while creating this FUNCTION

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MySQL

CREATE FUNCTION fn_getAdminNStaffNameByUserCode(
    _userCode varchar(100)
    ) RETURNS varchar(1000) CHARSET utf8
BEGIN
    DECLARE _staffName VARCHAR(1000) ;

    SET _staffName = (SELECT CONCAT(firstName,' ',lastname,' ',middleName) staffname  FROM staff where userCode = _userCode)  UNION
(SELECT username FROM usersmaster where usertype = 'admin' and usercode =_userCode) ;

    RETURN _staffName ;
    END;

Error:

SQL execution error #1064. Response from the database:

You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use
near 'UNION
(SELECT username FROM usersmaster where usertype = 'admin' and
usercode =' at line 7