Causes the excerpt to be required before a post can be published. Supports both editor types (classic and block aka gutenberg).

Noted That:

To install correctly this mandatory-excerpt.zip.
Fisrt Download the mandatory-excerpt.zip to your computer
Extract/Open mandatory-excerpt.zip to Your Computer.
Then, Find readme.txt file inside mandatory-excerpt.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "mandatory-excerpt.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "mandatory-excerpt.zip" Downloaded plugin from your computer, Where you downloaded mandatory-excerpt.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Mandatory Excerpts for WordPress" folder is added on left sidemenu. Now, Click on "Mandatory Excerpts for WordPress" folder.

Noted that: If you do not see "Mandatory Excerpts for WordPress" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Mandatory Excerpts for WordPress Configurtions and Settings or How to work "Mandatory Excerpts for WordPress" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Mandatory Excerpts for WordPress" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Mandatory Excerpts for WordPress
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide

Guide
Either use the WordPress Plugin Installer (Dashboard > Plugins > Add New, then search for “mandatory excerpt”), or manually as follows: Upload the entire `mandatory-excerpt` folder to the `/wp-content/plugins/` directory DO NOT change the name of the `mandatory-excerpt` folder Activate the plugin through the ‘Plugins’ menu in the WordPress Dashboard Note for WordPress Multisite users: Install the plugin in your `/plugins/` directory (do not install in the `/mu-plugins/` directory). In order for this plugin to be visible to Site Admins, the plugin has to be activated for each blog by the Network Admin.

Either use the WordPress Plugin Installer (Dashboard > Plugins > Add New, then search for “mandatory excerpt”), or manually as follows:

Upload the entire mandatory-excerpt folder to the /wp-content/plugins/ directory
DO NOT change the name of the mandatory-excerpt folder
Activate the plugin through the ‘Plugins’ menu in the WordPress Dashboard

Note for WordPress Multisite users:

Install the plugin in your /plugins/ directory (do not install in the /mu-plugins/ directory).
In order for this plugin to be visible to Site Admins, the plugin has to be activated for each blog by the Network Admin.

types, editor, classic, block, gutenberg, both, supports, required, excerpt, before,

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Reffered: https://wordpress.org/