Problem:
The parent Select Field in a given Child Post Edit screen is not available. What can I do, if I am sure, the parent Post type actually holds some posts?
Solution:
1. Head to Types > Post Types > your_custom_post_type > edit and ensure the Relationships is correctly set.
2. Save the Post Type again
3. Check your Product Edit Screen.
If now you can not see the Parent posts to choose, please follow:
1. List all products, check them all, choose "Edit" from the bulk edit mode, hit apply and save them, without editing any.
2. Do the same with the parent Post Type
If now still it does not work, check if the issue also persists with a WordPress Default Theme and NO Plugins BUT the Toolset Plugins?
If not, could you then re-enable the Plugins one after the other, and check the issue each time you enable a plugin?
Please report us when the issue comes back
It might also be due to the Theme.
Please do reactivate your Theme only after you are sure the issue isn't coming from a 3rd Party Plugin.
Relevant documentation:
https://toolset.com/documentation/user-guides/creating-post-type-relationships/