ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes
Solved : 

I solve this problem, by creating new table like this spool. when we want to create a table with utf8mb4 COLLATE utf8mb4_unicode_ci character set we can not use varchar(250) because 250 * 4 = 1000 (4 byte per character in this new encoding). 1000 is more than max key 767 byte for this encoding. so I create a table spoolll with varchar(100) :

CREATE TABLE `spoollll` (
  `username` varchar(100) NOT NULL,
  `xml` text NOT NULL,
  `seq` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  UNIQUE KEY `seq` (`seq`),
  KEY `i_despool` (`username`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=23492667 DEFAULT CHARSET=utf8 


and now I change the character set : 

mysql&gt; ALTER TABLE spoollll CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0


I would appreciate if someone comment on it and complete this issue.