Please check if under WPML > Settings, WPML > Translation Management or WPML > Translations, do you see a dialog that indicates that WPML has been migrated?
The WPML translation editor cannot function until this dialog has been completed.
Please also check that all users who are allowed to use the translation editor have a valid first and last name in the WordPress profile.
Verify which owners are enabled to use the editor at WPML > Translation Management > Translators. Then go to the Users section of WordPress and make sure that all those translators have a first name and last name in their profile.
If the problem still persists after that, please try to register WPML on the website again.
You will need to remove the site URL from your WPML account and then re-enter it to get a new site key, which you can then use to re-register WPML in WordPress.
