Thank you for your help.
I am a beginner. Please help if you understand.

Create a home page with your own domain (https://example.com/)
I installed WordPress 5.4.1 in a subdirectory and created a blog page (https://example.com/blog/).

I would like to use multi-sites to separate the blog categories that were separated.
Example) Blog https://example.com/blog/
Latest information https://example.com/news/

The top layer is made up of html, and what I want to multi-site is the layer below it.

While referring to other sites, after writing to wp-config.php for multi-site conversion,
The top page (https://example.com/index.html) in the upper hierarchy became invisible due to a database error.
Other higher-level pages (such as https://example.com/hoge.html) will be displayed.
You can see the blog site, but the address is https://example.com/blog/blog/.

The blog design is also an original one, but I am creating it.
After the multi-site, some images were not displayed.
After that, if you restore the wp-config.php and .haccess to the backup
The top page is displayed, but the image of the blog page is not displayed.
The path to the image is correct when I checked it with the browser source.

Source code
wp-config.php

define(‘SUBDOMAIN_INSTALL’, false);
define(‘DOMAIN_CURRENT_SITE’, ‘example.com’);
define(‘PATH_CURRENT_SITE’, ‘/blog/’);
define(‘SITE_ID_CURRENT_SITE’, 1);
define(‘BLOG_ID_CURRENT_SITE’, 1);

I checked the database name, user name, host name and password in wp-config.php
There were no mistakes.