CREATE TABLE  `wp_commentmeta` (

 `meta_id` BIGINT( 20 ) UNSIGNED NOT NULL AUTO_INCREMENT ,
 `comment_id` BIGINT( 20 ) UNSIGNED NOT NULL DEFAULT  '0',
 `meta_key` VARCHAR( 255 ) COLLATE utf8mb4_unicode_ci DEFAULT NULL ,
 `meta_value` LONGTEXT COLLATE utf8mb4_unicode_ci,
PRIMARY KEY (  `meta_id` ) ,
KEY  `comment_id` (  `comment_id` ) ,
KEY  `meta_key` (  `meta_key` ( 191 ) )
) ENGINE = INNODB AUTO_INCREMENT =26 DEFAULT CHARSET = utf8mb4 COLLATE = utf8mb4_unicode_ci;

MySQL meldet: Dokumentation

#1273 - Unknown collation: 'utf8mb4_unicode_ci'