capthong

Problem solved

I have WP multisite installed under a sub-directory and redirect the sub-directory to the root.

Referring to

I have the .htaccess under Public-html folder as

RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www.)?YourDomain.com$
RewriteRule ^(/)?$ blog [L]]

Under the subdirectory, I have the .htaccess as:

`RewriteEngine On
RewriteBase /writing/
RewriteRule ^index\.php$ – [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ – [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

And it works.