Welcome to the WPML support forum. I will do my best to help you to resolve the issue.
For WordPress sites, the recommended charset is utf8mb4 and the recommended collation is utf8mb4_unicode_ci. Please refer to the following documentation for more details.
So please verify the following and let us know your feedback.
1) Open the wp-config.php file of your site root folder for editing and check if you have something similar to the following code.
define( 'DB_CHARSET', 'utf8' );
2) Maybe in the database there are some tables with different collations and it could be the reason for the warning(they usually start with utf8mb4 like utf8mb4_general_ci for example). So please login to the database using a tool like PHPMyAdmin and check if some tables have a different collation or not.
