Can't access to wordpress multi-site wp-admin

Problem:

You have setup the standard Wordpress multi site configuration in Wordpress ( using "directories" not subdomains ) and create one or more sub sites; the problems is that when you try to access to the sub-site web view or sub-site wp-admin you get a 404 error and it's not possible to view the template styles files or login into the wp-admin to manage the sub site.

Solution:

you have to apply a different configuration in the .httpaccess file, just apply the next configuration:

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

Source:

Wordpress httaccess Support:
https://wordpress.org/support/article/htaccess/