MultiNetwork plugin with parked domain doesn't work.

Hi all, I’ve set up a WordPress Network for serving all of our site (with WP Multi-Network plugin) within one installation. To do that I am adding my domains to parked domain section of my server, and configure settings in WP Multi-Network plugin. But some of site are not ready to make them public (under construction), and for this purpose I’ve set up our main site (www.main-site.com) as a “Under Construction” page (with WP Maintenance Mode plugin) and I would like to point other sites (not ready to public ones – e.g “www.under-construction.com” ) as parked domain so when someone enter this site, he/she will see the content of main site (www.main-site.com) which shows “this site is under construction”. But to do that, when I add “www.under-construction.com” as “parked domain”, url shows me ” Error Establishing a Database Connection” message. How I can solve this problem? Is it about DNS problem, cache problem, redirection problem, etc.. Thanks.

My htacces file is same as below

# BEGIN iThemes Security - Do not modify or remove this line
	# Disable Directory Browsing - Security > Settings > System Tweaks > Directory Browsing
	Options -Indexes

	<IfModule mod_rewrite.c>
		RewriteEngine On

		# Disable PHP in Uploads - Security > Settings > System Tweaks > Uploads
		RewriteRule ^wp\-content/uploads/.*\.(?:php[1-6]?|pht|phtml?)$ - [NC,F]
	</IfModule>

	# Enable the hide backend feature - Security > Settings > Hide Login Area > Hide Backend
	RewriteRule ^(/)?wp-admin/?$ /wp-login.php [QSA,L]
	RewriteRule ^(/)?wp-register-php/?$ /wplogin?action=register [QSA,L]
# END iThemes Security - Do not modify or remove this line

# BEGIN [SEO_REDIRECTION]
 ErrorDocument 404 /index.php?error=404
# END [SEO_REDIRECTION]

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}/$1 [R=301,L]

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
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]
</IfModule>

# END WordPress

# Wordfence WAF
<Files ".user.ini">
<IfModule mod_authz_core.c>
	Require all denied
</IfModule>
<IfModule !mod_authz_core.c>
	Order deny,allow
	Deny from all
</IfModule>
</Files>

# END Wordfence WAF

<Files wp-config.php>
order allow,deny
deny from all
</Files>

<Files .htaccess>
order allow,deny
deny from all
</Files>

<Files 403.shtml>
order allow,deny
allow from all
</Files>

NOTE: Other sites that ready to public (online ones) are working correctly.