Я не могу воспроизвести проблему:
MariaDB [_]> DROP PROCEDURE IF EXISTS `sp_createUser`;
Query OK, 0 rows affected (0.001 sec)
MariaDB [_]> DELIMITER $$
MariaDB [_]> CREATE DEFINER=`root`@`localhost` PROCEDURE `sp_createUser`(
-> IN `p_name` VARCHAR(20),
-> IN `p_username` VARCHAR(20),
-> IN `p_password` VARCHAR(20)
-> )
-> BEGIN
-> IF ( select exists
-> (select 1
-> from tbl_user
-> where user_username = p_username) ) THEN
-> SELECT 'Username Exists !!';
-> ELSE
-> INSERT INTO `tbl_user`
-> (
-> `user_name`,
-> `user_username`,
-> `user_password`
-> )
-> VALUES
-> (
-> `p_name`,
-> `p_username`,
-> `p_password`
-> );
-> END IF;
-> END$$
Query OK, 0 rows affected (0.000 sec)
MariaDB [_]> DELIMITER ;