I ran a full test on this testing environment here (hidden link) and I was able to see the "Ninja Forms" option in the dropdown.
You can click the link to automatically log into the testing environment and see that we are using there the latest WPML main plugin and the latest version of the String Translation plugin.
Probably there's a plugin that is creating that issue or it might be a local caching-related issue.
I hope that you will find this information helpful.
