If you need to put your WHMCS website into maintenance mode you can use the advanced version included in WHMCS Firewall.

Login to your WHMCS > hover over the menu tab ADDONS > click on FIREWALL MANAGEMENT > hover over the FIREWALL TOOLS menu tab > click MAINTENANCE MODE

WHMCS also has its own generic maintenance mode built in.

Login to your WHMCS > hover over the menu tab SETUP > click on GENERAL SETTINGS > check the MAINTENANCE MODE option

Whats the difference?

The WHMCS Firewall maintenance mode feature gives you more options for configuring the style of your maintenance mode message, images, page and notifications.