@luismiguelopes - If your site is down as a result of the .htaccess configuration, you can always backup your .htaccess (download a copy) and then delete it from your web root after commenting out the define lines in wp-config.php. Once you’ve done this, refresh the page and/or attempt to access yourdomain.ext/wp-admin and a new .htaccess should be generated for you (if not, clicking “save” on the permalinks page will regenerate it for you).

When commented out, the lines should look like:

//define( 'MULTISITE', true );
//define( 'SUBDOMAIN_INSTALL', false );
//define( 'DOMAIN_CURRENT_SITE', 'yourdomain.ext' );
//define( 'PATH_CURRENT_SITE', '/' );
//define( 'SITE_ID_CURRENT_SITE', 1 );
//define( 'BLOG_ID_CURRENT_SITE', 1 );

Once you’re ready to reattempt the installation, the following should be set in wp-config.php

define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', false );
define( 'DOMAIN_CURRENT_SITE', 'yourdomain.ext' );
define( 'PATH_CURRENT_SITE', '/' );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );

Ideally, yourdomain.ext should be setup without the www. attached (as shown in the example above).

For the .htaccess file, try:

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# Add Trailing / 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]