Problem:Unable to register live site key on site hosted with Pantheon
Solution:
They do have instructions: https://docs.pantheon.io/plugins-known-issues?search=API#wpml---the-wordpress-multilingual-plugin
However those instructions only work if you DON'T register WPML on the development site before pushing the plugin to the live site.
The correct process would be to install WPML on the dev site, then push it to live. After that create WPML keys for both dev and live. Then activate the plugin on both sites, that would allow you to enter the appropriate keys.
If you install and register WPML on the dev site, make your translation then clone to live (just pushing code changes doesn't include the database). You do have to use the define( 'OTGS_INSTALLER_SITE_KEY_WPML', '' ); for the live site in wp-config.
Unfortunately, because of the way Pantheon is setup that means you can only make translations on the live site. If you want to create translations on the dev site then you'd need to modify wp-config again for the dev key.
One other item: Since Pantheon is using alias URLs I had to first go to create a key for the actual site URL (example.pantheon.io) and modify wp-config. Then modify wp-config again with the live URL key (example.com). Then in the error message in the WP dashboard go through the steps confirming that the new URL is example.com. Finally going to example.com and also confirming there that that URL was the correct one.
Again all these steps are only if you don't follow the correct install/register process for Pantheon's environment (which isn't really intuitive and they don't mention possible issues like these).
Hopefully this makes sense and might possibly help others.
Relevant Documentation:
