Resolved in: WPML 4.0.7
When you create a Course using LearnDash plugin and translate it, the translated page will display a 404 error.
Navigate to WPML -> Translation Managment go to the Multilingual Content Setup tab and click on the Custom posts slug translation options link. Select the Translate custom posts slugs (via WPML String Translation) option and click the Save button as illustrated below.
Now that we have set the custom post slugs to be translated, scroll down till you reach the Custom posts section. For the custom post type Courses select “Use different slugs in different languages for Courses” option, add your translated text and Save as illustrated below.
Note: Make sure that you set your permalinks to be Post name. You can do this from your WordPress Dashboard by navigating to Settings -> Permalinks and choose Post name then click Save.
You can follow the same steps to resolve displaying a 404 error in the secondary language for other LearnDash custom post types.