Overview of the issue
We have noticed that there are some customers reporting a fatal error on activating WPML
PHP Fatal error: Call to undefined method Composer\\Autoload\\ClassLoader::setPsr4() in .../wp-content/plugins/sitepress-multilingual-cms/embedded/composer/autoload_real.php on line 33
This issue happens from one of your other plugins using an outdated version of Composer, where the autoloader does not support
The problem happens when a plugin with such old version bundled is initialized before WPML plugin.
You can read more details here: https://github.com/composer/composer/issues/2553
To fix the issue you must first find the offending plugin: you can do it by disabling all plugins, except WPML, and then re-activating them one by one.
Then it is advisable that you seek updated version of that plugin: if the issue does not get resolved, contact the author and let him know about the issue.