can you share with us the plugin that you are trying to install ?
in this way we can better try to answer you
thanks

2 files
ymages.xml

<?xml version="1.0" encoding="utf-8"?>
<extension version="3.0" type="plugin" group="system" method="upgrade">
    <name>System - Ymages</name>
    <author>Joomla! Project</author>
    <creationDate>February 2020</creationDate>
    <copyright>Copyright (C) 2005 - 2020 Open Source Matters. All rights reserved.</copyright>
    <license>GNU General Public License version 2 or later.</license>
    <authorEmail>web@ymages.com</authorEmail>
    <authorUrl>ymages.com</authorUrl>
    <version>1.0.0</version>
    <description>custom library.</description>

    <files>
        <filename plugin="ymages">ymages.php</filename>
        <filename>index.html</filename>
    </files>
</extension>

ymages.php

<?php
/**
 * @copyright   Copyright (C) 2005 - 2020 Open Source Matters, Inc. All rights reserved.
 * @license     GNU General Public License version 2 or later.
 */

defined('_JEXEC') or die;

/**
 * Ymages plugin class.
 *
 * @package     Joomla.plugin
 * @subpackage  System.ymages
 */
class plgSystemYmages extends JPlugin
{
    /**
     * Method to register custom library.
     *
     * return  void
     */
    public function onAfterInitialise()
    {
        JLoader::registerPrefix('Ymages', JPATH_LIBRARIES . '/ymages');
    }
}

the plugin is installed and works but I get this error

I could install on both, J3.9.15 and J4 without error

Use <name>plg_system_ymages</name>
and remove line <filename>index.html</filename>.

Then I have to try a clean install

thanks

Closed as not a core joomla issue