Mattermost Boards (formerly known as Focalboard) is included in the Mattermost Cloud workspace, enabled by default, and upgraded automatically.
For self-managed deployments, Focalboard is available in the Plugin Marketplace.
As a System Admin, go to Main Menu > Plugin Marketplace.
Search for Boards.
Select Install if not yet installed, then select Configure to enable.
From the plugin configuration page, set Enable Plugin to true.
Select Save to enable the plugin.