The problem is that you set DELIMITER to be //, but later using ; as DELIMITER. After changing it to //, ; is a simple unrecognized character...
If you have no special reason to change the default DELIMITER do not! But if you did use the new one...
DELIMITER //
CREATE PROCEDURE Sample
BEGIN
DECLARE v_SQLSTR VARCHAR(800) //
SET v_SQLSTR='Hi' //
END //
DELIMITER ;