Solution:
There was a .htaccess in the wp-admin folder that caused the issue.
So, in such cases, please check if there is a .htaccess in wp-admin folder and delete it.
You can also check the main .htaccess file for your website in the main directory.
Also, you may disable all non-Toolset plugins and activate the default theme to test the issue.
//Please take a backup of your website before proceeding with any of the previous steps.
Problem:
Can I select a Child Post from the Parent Post Edit interface, and if no, how can I hide the Child Post section in the parent post edit interface.
Solution:
You can not select a Child Post Type when editing the Parent.
This is only possible the other way around.
When you create a Child Post, you can (after you save it) select a Parent.
From the Parent Post Edit Screen you can only create new Child Posts.
You can remove the "Create Child Table" by following this steps:
1. Types > Post types > your_parent_post_type > edit > Post type relationship > Child Post types
2. Open the link next to the selected Child Post Type
3. Check "Do not show management options for this post type"
Relevant Documentation:
It does output the images in front-end, since it does work, you can ignore the red warning, since it is just only for help you format the codes.
